Add disabled information to labelfile

Indicates whether a label is disabled in the labelfile. This indicates
whether it should be considered for printing under normal circumstances
(not explicitly requesting all labels) for printing and such.

Disabling a label allows the user to keep certain labels in the
labelfile without being bothered by outdated labels in listing output
and the like.

Represent the disabled state in the list header with "X".
Don't print disabled labels and add --all switch

Don't print labels that have been disabled.

Add an `--all` switch to the `dispass list` command which allows listing
of all labels regardless of their disabled state.
Add disable/enable commands
Update documentation
Merge remote-tracking branch 'ryuslash/disable'