Adds editorconfig and makes plugin.json style consistent

[bdharrington7]

adds newlines at end of file, etc.

[julianlam]

Why 2-space ?

[bdharrington7]

It was arbitrary, I've seen more json (and for that matter, javascript) files with 2-space indentations, but in this repo, since the *.js files were tab-indented, I conformed to that style. as for the *.json files, one was 2-space indented (package.json), but plugin.json was tab-indented. Which do you prefer?