hold all phrases in the middleware.

[guillaumearm]

for v1.0

from #8, need close #16 before,

we want all phrases and his corresponding locales stored inside the middleware :

• add a fourth optional parameter to createPolyglotMiddleware() :

const defaultOptions = {
  cache: false,
};

• add a @@polyglot/SET_OPTIONS actions for dynamically set the cache mode.
• add a setCache() action creator.