[RFC] Some options should be 'common', not 'global'

[Ian Clatworthy]

In discussion on IRC today with Aaron re the nuances of implementing
noise levels for commit, we came to the conclusion that there may be a
good case for 3 types of options:

* global - parsed specially with resulting limitations (e.g. can't be
  used in aliases)

* common - options always allowed for all commands, even if commands
  decide to ignore them

* command-specific - as now.

In other words, we want to introduce "common" options over and above the
existing stuff. Unlike global options, these would be supported in
aliases and commands could provide custom help text for them.

The first set of common options would be an enumeration of noise levels,
namely --quiet (-q), --normal (-n?), --verbose (-v).

In terms of implementation, my expectation is that --quiet should be
implemented at the bzrlib.trace level, while verbose would be a boolean
flag passed to all commands.

Does this sound a good idea to everyone? If so, does anyone have any
concerns about backwards compatibility given the implementation proposal
above?

Ian C.