Update mu4e-context-switch to not call the enter/leave functions when
'changing' to the current context. However, calling those functions (if
defined) can be force through a prefix arg. Document this.
Make mu4e-context-determine recognize 'policies', i.e. what to do when
no context matches. This part is WIP.
When contexts have been defined, automatically select one at startup --
either the first whose match-function returns non-nil, or simply the
first one.
Document this, too.
Define mu4e-context - a structure with settings and enter/leave
functions for contexts, and some functions for switching between
contexts and auto-detecting them.