feat: allow context to be stored in location export

[crisidev]

I have created a LSP server based on Bacon (great software btw) called bacon-ls (blog post) that makes use of the location export.

This PR allows a new setting called add_context_to_message changing the line export to include all the context from the compiler.

[Canop]

Rather than a add_context_to_message setting, I'd prefer to allow changing the line_format to

line_format = "{kind} {path}:{line}:{column} {message}{context}"

The purpose of the line_format parameter is to allow selecting and partially format what's in the exported line.

This doesn't prevent optimization: we can check whether the format contains {context} before calling extract_diagnostic_context.

If you prefer I do the change, I can do it.

[crisidev]

This is fine by me, it achieves the same result and it's a better approach. If you want to implement the change, be my guest :)

[Canop]

Follow up: #188

[crisidev]

Superseded by #188