This is a common ugly pattern we make our user do:

d = self['key']
self['key'] = d

Now they can just do that:

with self.mutable('key') as d:
   # do something on d

And d will be saved automatically in the key entry at the end of the
scope.

A lot of crap all around: try on imports / if PY2/Py3, tests on unicode,
src conversion, travis config, docs ...

Those new CLI parameters allows the user to manipulate the plugin
configuration, states but also tweak errbot internals.

Fixes #608.

If there is already a new db, warn the user so s/he can solve that
manually.

Fixes #686.

- this is triggering a deprecated warning (it was not meant to be used
  in the first place).
- move self.store to self._store so it won't happen again on self.store.

We will reuse the same manager to load backend and storage plugins.