let user decide if 'run all' behaviour should be enabled

[leemhenson]

I'm adding guard-rubocop to a large project that has a lot of rubocop failures. Running rubocop from the terminal grinds for a long time, so I want to control guard-rubocop so it only ever runs on changed files and never runs on the entire project.

[yujinakayama]

You can prevent the guard-rubocop's “run all” on Guard startup by specifying all_on_start: false in Guardfile, and guard-rubocop won't run on the entire project unless you type ↩, a or all in the Guard console. If you want to prevent the “run all” on ↩, a or all (it seems you do), I think you should submit a feature request to Guard itself, not to each plugin.

[coveralls]

Coverage remained the same when pulling aec9607 on musicglue:master into 4e9fcee on yujinakayama:master.