Several changes regarding packaging, namespacing and handler API:

 *  Utility function module is now called 'imapdomo', not 'common'.

 *  The module behaves as one, instead of being just a global-modifying
    include.

 *  Lots of cleanups as advised by luacheck (local declarations,
    removal of unused variables...)

 *  Handlers now have to access utility API via `require` and provide
    accout configuration in new file mailboxes.lua, which has to return
    table.  Accessing mailboxes is via utility function.  (This sounds
    complicated but actually is not.  Believe me! :))

Dead since 919fa12, ie. v0.0.4

Having just 4 valid actions (albeit they do form a meaningful workflow)
was rather historical limitation caused by author's negligence.
User should be able to define any actions they want by simply creating
handlers for them.

Later we might want to provide a guidance (template) for that, but
that's a TODO item.

The host split turned to be a vast overengineering.