[Fix #123] Teach the ruby checker about warnings

The warnings generated by the interpreter always start with `warning: `,
unlike errors that have no prefix.
Fix docstring

`flycheck-mode` nowadays always enables the mode.

[ci skip]
Require appropriate feature for global mode
Replace `should` with `should-not`
Inhibit checkdoc in Carton files

Carton files are no real libraries, so they don't have to follow the
documentation rules.
Fix markup

[ci skip]
Add missing customization group
Move faces into a separate customization group
Remove obsolete variable
Move fringe icon into separate function
Make error indication customizable
Replace association list with a simple cl-case
Simplify hook setup and teardown
Get rid of deferred checking

This deferred checking stuff was just a super-complicated way to check
only visible buffers.  So let's just do this right away: Inhibit all
syntax checks in invisible buffers and recheck buffers if they become
visible again.
Unbreak Rubocop tests

Grr, @bbatsov ;)
Fix byte compiler warning in Rubocop tests
Get rid of superfluous warning
Simplify some conditionals
Clean up requires
Make optional google-this library available for development