Make --only enable the given cop

It was said in #900 that it is a bug for --only to not enable the cop.
Merge pull request #915 from jonas054/improve_only_option

Make --only enable the given cop
Fix default exclusion of vendor directories

Excluding directories from a .rubocop.yml file works, but before this
fix, the setting of AllCops/Excludes in default.yml didn't have any
effect.
Merge pull request #917 from jonas054/fix_vendor_exclude

Fix default exclusion of vendor directories
fix hanging while auto correct

fixes #908
Merge pull request #918 from hiroponz/fix-hang-while-auto-correct

fix hanging while auto correct
Add auto-correction to Not cop
Add auto-correction to ClassMethods cop
Unify inclusion/exclusion params names

They were Includes/Excludes for AllCops, but Include/Exclude for
individual cops. Now they are Include/Exclude everywhere for sanity's sake.
[Fix #888] Require only the powerpack extensions that we actually need
Add auto-correction to StringConversionInInterpolation cop
Extract NonNilCheck cop from NilComparison
Fix offenses reported by NonNilCheck
Add auto-correction to NilComparison cop
Simplify a bit NilComparison
[Fix #919] Remove auto-correct avoidance in HashSyntax

We have an auto-correct loop that deals with clobbering errors now, so
instead of avoiding a correction we just make the correction range a
bit larger.
Remove a couple of redundant spec examples
Refine a couple of specs
Add auto-correction to NonNilCheck cop
Merge pull request #920 from jonas054/fix_only_hash_syntax_autocorrect

[Fix #919] Remove auto-correct avoidance in HashSyntax