Remove the item about React editor in the contribution guideline

Non-React editor is no longer available.
Replace Config::observe with ::onDidChange

This fixes #114.
Fix error on parsing result of flake8

flake8 may produce unformatted line as:

test.txt:1:17: E901 SyntaxError: invalid syntax
    Check if the syntax is valid.
Fix possible error on parsing Checkstyle format XML

This fixes #113.
Update changelog for 0.20.1
Prepare 0.20.1 release