Add WebSocket server

[MatejKastak]

Description

Implementation of WebSocket server. This PR allows users to start the WebSocket server on a given host and port that will "wrap" the LSP protocol. The interface is the same as other SERVER.start_* methods.

The main use case is to create convenient interface to interact with editors that are embedded in internet browsers (for example Monaco LSP-client), this could potentially allow for broader adoption.

Related to #85

Code review checklist (for code reviewer to complete)

• Pull request represents a single change (i.e. not fixing disparate/unrelated things in a single PR)
• Title summarizes what is changing
• Commit messages are meaningful (see this for details)
• Tests have been included and/or updated, as appropriate
• Docstrings have been included and/or updated, as appropriate
• Standalone docs have been updated accordingly
• CONTRIBUTORS.md was updated, as appropriate
• Changelog has been updated, as needed (see CHANGELOG.md)

[MatejKastak]

I think PR is at the point when it's ready for review/discussion.

1. Adding websockets as a dependency made some of the tests fail, how would you like to resolve this problem?

   • websockets lib should work on >= python 3.6 based on their docs (We can probably make 3.6 pass)
   • what about python 3.5? You probably want to keed support for 3.5
   • we can make WebSockets feature not default but rather opt-in, and let users install it via pip install pygls[websockets] for example. After make some import guard to start_websocket and throw runtime error if the package is not found. The tests for this variant would run only on >= 3.6.

2. How to handle pathargument - code

   • we can add optional or madnatory argument to start_websocket for user to specify exact path on which the server should listen
   • we can discard it and listen on all endpoints

Any feedback appreachiated.

[danixeee]

@MatejKastak Sorry for the delayed response. We have removed support for python 3.5, so that's not a problem anymore. Take a look at the inline comments and let me know what do you think about how we should solve them.

[danixeee]

@MatejKastak I just pushed minor updates, thanks and sorry for waiting this long.

[danixeee]

Works fine with Monaco client.