[WIP] Added List labels and milestones command; Added Label command; Some refactoring
Added conditions based on the options; Added labels validation.
Added cache files per issues/pr and also by organization/repository
Added "current labels" of an issue to the output. Add possibility to add
multiple labels separated by commas.
Fixed typo
Merge pull request #12 from danielcsgomes/new-commands

[WIP][New Commands] Label issues, List milestones, List labels