Make JCommander an optional dependency (#317)
Make JCommander an optional dependency (#317)

Related: #306
Motivation:

When a user pulls `centraldogma-testing`, it also pulls JCommander
transitively, but it's actually of no use.

Modifications:

- Make JCommander optional

Result:

- Less transitive dependencies
line/centraldogma,line/centraldogma,line/centraldogma,line/centraldogma,line/centraldogma