Remove refinerycms-i18n from engine Gemfile template because it will be installed automatically.
plugin -> extension.
Improve messages when running refinerycms command with various options.

This also:

* shows help message when no arguments are specified
* adds -h (for --help) shortcut to see help message
* adds -v, --version to see current gem version
Merge pull request #2348 from refinery/improve_refinerycms_executable_messages

Improve messages when running refinerycms command with various options.
Fix #1915 - The `span` element containing a `style` tag causes markup corruption

https://github.com/wymeditor/wymeditor/issues/211
Add note to changelog about #2350.
Update "what's changed guide" with note about #2331.
Merge branch 'master' of https://github.com/refinery/refinerycms