Add auto-indent when pasting

Currently, auto-indent is not used when pasting. This makes sense for coffeescript and other indentation-sensitive languages, because in those languages, auto-indent would often make the indentation worse. This adds a separate config option, `autoIndentOnPaste` that allows auto-indentation to be used when pasting.

Closes #1407

* [x] auto-indent pasted text
* [x] in `::suggestedIndentForBufferRow`, base indentation on previous non-blank line
* [x] don't auto-indent when pasting after non-whitespace characters
* [x] make a separate config option, `editor.autoIndentOnPaste` for auto-indenting pasted text
* [x] add a description to the original `autoIndent` config option, differentiating it from `autoIndentOnPaste`
What do you guys think of this description?