New feature requests for lazy loading

[Shougo]

1. call denops#plugin#discover() should not reload existing denops plugins

2. Add g:denops#ready like variable to check DenopsReady is already called.

They are needed to support lazy loading in plugin managers.

[lambdalisue]

I'll think about it 👍

[lambdalisue]

#76

[Shougo]

It is already merged.
I think it can be closed.