Add UI/UX for setting the binary path

[AlexCannonball]

Hello, I'd like to propose UI/UX improvals for users that have just installed the extension, or downloaded the protolint and have no softlink to the binary in the OS.

Changelist:

• Limit the scope of protolint.path configuration, as it's usually just one per machine.
• Add the clickable repository link for Settings UI.
• Add the warning message when the default path isn't working. Buttons:
  • Open the repository link in the browser.
  • Find the binary via the system file picker (it requires vscode engine ^1.31.0).
• Add the key binding ALT+SHIFT+L for protolint.lint command.
• Event subscription refactoring, make the necessary extension disposables to be cleared after the extension deactivation.
• Refactoring for the target language filter (use DocumentSelector).
• Add the diagnostic source protolint:
  • 
• Fix a bug when linter warnings were not cleared after closing the file. I guess if you get this state and edit the file in another editor and then open again in vscode, you'll see old irrelevant warnings.
• Fix a bug when linter warnings are kept after changing the file language from protocol buffer to another language.
• Experimental fix for #32 using IPC + child_process.execFile.
  • Tested on Windows 10 only. Testing on Unix/macOS would be appreciated.
  • Works slower than linting a file (takes ~1-2 seconds). Probably because protolint makes 21 excessive readings from the socket and there's no cache for IPC.
  • At the moment IPC is used only when the document has unsaved changes.

If you're happy with the changes, we need to update readme.