Keep line length to 80

Using `:vimgrep /.\{81\}/ config/**/* lib/**/* spec/**/*`.

- Enable `LineLength` cop again for such spec files
Ignore StyleGuide from configuration parameters
Add more style guide references to cops

Related to #824.
Merge pull request #1306 from yous/patch-explanation

Add more style guide references to cops
Merge pull request #1303 from yous/patch-line-length

Keep line length to 80
Fix auto-correction `RedundantBegin` cop

Auto-correction of `Style/RedundantBegin` cop have treated new line as
indentation and deleted them.

Fix #1307.
Make PredicateName more flexible

It can now report offense for a name like `has_something` without
necessary suggesting the name `something?`.
Merge pull request #1308 from yous/patch-redundant-begin

Fix auto-correction `RedundantBegin` cop
Fix changelog entry