Update Utils class camelCaseToUnderscore method

* Add self when calling REGEX_CAMEL_CASE_UNDER const on
  camelCaseToUnderscore method
Fixes & cleanup to GeneratorPluginImageEffectCommand.php

* Update parameters description
* Add multi-line on ask methods:
  * Enter the plugin name
  * Enter the plugin label
* Relocate call to  Utils::camelCaseToUnderscore
Fix & cleanup to PluginImageEffectGenerator

* Update generate docblock add missing parameters
Fix & cleanup to plugin-imageeffect template

* Update comment message
* Remove blank line
Merge pull request #97 from jmolivas/image-effect-fixes

Image effect fixes