Make plugins "more optional"

[gatesn]

Right now, although it's possible to turn off various plugins (e.g. pydocstyle, rope completions etc.) the pydocstyle and rope dependencies are still required.

We should move towards having all plugins optional, beyond Jedi, and therefore only used if they are installed. This would require the module imports to be moved within plugin scopes, and probably some runtime version checks, e.g. ensure that pyflakes is >= 1.6.0 at runtime, else loudly print out a warning.

[wmonk]

Can a requirement of this feature be to be able to turn them off from the cli?

[gatesn]

@wmonk can we track that here? #300