Add a highlight group for denoting bad whitespace.  Also added a match rule for leading tabs in a line when in Python mode.
Add a highlight group for denoting bad whitespace.  Also added a match rule for
leading tabs in a line when in Python mode.

Also fixed some grammatical errors in the comments.

sk-/python2.7-type-annotator,sk-/python2.7-type-annotator,sk-/python2.7-type-annotator