Implement web links and custom link matcher registration

[Tyriar]

Fixes #455

🆕 Web links

This PR adds web links to the terminal which can be clicked to open the URL, a custom handler can be attached which will change the links to use an onclick handler instead.

Terminal.prototype.attachCustomKeydownHandler = function(customKeydownHandler) { ... }

🆕 Custom link matchers

The ability for consumers to register custom link matchers has also been added, these are basically the same as web links but can match literally anything the consumer wants.

Terminal.prototype.registerLinkMatcher = function(regex, handler, matchIndex) { ... };
Terminal.prototype.deregisterLinkMatcher = function(matcherId) { ... };

Implementation details

• Web links use the old linkify addon's strictUrlRegex, meaning only http(s) protocol prefixed links will activate
• Linkification should happen 200ms after a row was last updated, provided no other refresh occurred at which point the timer would be reset. With the default styling of mouse over underline, users likely wouldn't even notice this delay.
• Link matchers are evaluated in the order in which they are added, starting with the builtin web link handler. This will likely need to change later and support priorities as generally new matchers should be higher priority but the http handler needs to beat out many of them of paths would be incorrectly matched.
• The web link handler cannot be deregistered.
• The link matcher API allows the regex match ID to be set, to allow greater flexibility.
• Testing should be easy after Add PhantomJS test support and CharMeasure tests #535 lands.

[parisk]

LGTM and works great for me.

Seems to be resolving #27 also.

[Tyriar]

@parisk I'm having second thoughts on how deregister/register works. While currently it's very simple to use, it could be made far more powerful if the link matchers were assigned to actual rows. Consider the following which would be a really cool experience:

1. Scan for ls line
2. Register an ls link matcher which will linkify all items in the output
3. Scan for end of prompt
4. Deregister ls link matcher

Currently the above will do not linkify anything as the link matcher is immediately deregistered. Before merging this I'm going to look at tighter integration between the linkifier and the rows, so instead of just discarding the link matcher completely on deregister, the rows where it was active would keep a reference to it (which would now live outside Linkifier) and eventually be discarded when all references to it are gone (when all rows that had that link matcher go off the end of the buffer and outside references are dropped).

[parisk]

I think I get it and indeed sounds powerful. Just a clarification question:

How will you register link matchers to lines (e.g. using line number, DOM element, something else)?

Also, will the linkifying happen automatically or the host application has to invoke it manually at will.

[Tyriar]

There would need to be another step where when a row is written to it assigns references to the current link handlers on lines. As for where exactly, thinking about it.

[parisk]

Do you plan to implement this in this PR or as an enhancement in a next one?

[Tyriar]

In a new one would probably be better provided we don't advertise the API yet as it could change.

[Tyriar]

Merging this, created the following follow ups: #549, #550, #551

[puzrin]

@Tyriar I'd suggest to take a look at https://github.com/markdown-it/linkify-it for link pattern search algorythms.

[Tyriar]

@puzrin thanks for the link!