Custom pull descriptions

[korthout]

Adds a new input configuration option: pull_description.

The value is used as template for the description of pull requests created by this action.
Placeholders can be used to define variable values.
These are indicated by a dollar sign and curly braces (${placeholder}).

The action.yml and example workflows simply refer to the README for all available placeholders.
The README contains a new subsection (Placeholders) under Installation, to specify the available placeholders and what they are replaced with.

closes #51

[negz]

Thanks @korthout! I sadly probably won't get a chance to test this out in until at least next week, but the implementation looks like exactly what I'd like.

[korthout]

@negz That's great to hear. I was planning on doing some additional testing today and then merging it, so I'm happy to hear you're positive about the config. Personally, I feel this feature also opens up the possibility to add more placeholders (e.g. related issues, something my team wants) without changing the current behavior for other users. So I believe this adds a lot of value. Thanks again for bringing up this idea.

[korthout]

I've tested it, and all works as expected: default + documented example.

You can also escape the hash character # with a backslash, like \# to please the GitHub yml editor. This does add the backslash to the actual description, but GitHub then simply ignores it when rendering the description. Personally, I prefer not escaping them.

I did note one small problem. Each placeholder can only occur once in the description. Only the first is replaced, the others are not. It's not really a bug, just the current behavior. Let's see if someone needs this at any point.