Plugin support

[jakirkham]

Would be good if this could support different launching methods that could be described in some plugin directory. Presumably this would be a package in the library. This could include some standard ones like SGE, LSF, MPI, DRMAA, multiprocessing, etc. as built-ins. However users could easily make others that are not supported and have them installed here too. Thus this package would detect available plugins at runtime. A subcommand could be used to specify which launcher to prefer (e.g. splauncher sge <...>). If one isn't specified, there could be a hidden directory in the user's home directory, which would specify the order to check/try. Can probably borrow some lessons learned from ipyparallel to inform how to do this.

[jakirkham]

Would add additional plugins could be added in the user's configuration directory. That way if they have some custom changes to the standard plugins, they can provide them there.

[jakirkham]

Was hoping to get a deprecation into this release to start this process. However it doesn't look like we have time. So going to punt on this for now.