Make open_browser take AsRef<OsStr>, add open_path function

[philippeitis]

Since open_browser is the underlying implementation of open, I think making it provide an AsRef<OsStr> interface is acceptable (per the previous PR). However, open by default will only take AsRef<str>, while the new function, open_path, will only accept AsRef<Path>.

With regards to tests, is there anything in particular you would like to see? Would it be appropriate to add a simple test file and have all supported systems try to open this file?

[amodm]

Closing this without merge, for the following reasons:

1. I'd already mentioned earlier in Make open_browser take AsRef<OsStr> #35 that the APIs cannot be based on OsStr. They have to based on str, as we can guarantee conversion from str to OsStr, but not vice versa. This PR does not adhere to that request.
2. While I was open to adding an open_path earlier, I realise upon thinking more deeply that the behaviour is going to be inconsistent. In some situations with the default browser, this will open up a file browsing application instead of the browser. As such, I'd like to wait for more feedback on it from the users before taking a call on it, as making future modifications to public facing APIs is a nightmare.
3. My current thought on AsRef after a bunch of research, is to not include it, and keep it as &str. I've documented the reasons here