changed all bundler options to be optional

[Kriechi]

setting one of the options to a nil-value will exclude it from the final
bundler command

based on the work of @maxmouchet

closes #3 #6 #8 #14

[Kriechi]

I've rebased and added a default value for the command to add additional binaries to the array (might be nil, so default to [])

this would also close #20 because the bin-mapping is in its own task with description.

[Kriechi]

@kirs any comments?

maybe we should try to cleanup most issues and PR from all capistrano-* gems, before Capistrano 3.1 gets released?

[deepsystm]

agree w/ @Kriechi

changed all bundler options to be optional!

[kirs]

The only thing I'm afraid of is backward capability. Maybe we should better describe it in the CHANGELOG?

[Kriechi]

I have added a section to the changelog.
The only thing breaking backward compatibility should be the bundle_dir rename to bundle_path.

[kirs]

Thanks!