v0.19 - Group Therapy

Error messages
Multiple error message are now supported.
In elvish and zsh these are shown separate from the completion values.

Usage message
Added Usage message which defaults to flag description / command usage.
This is only supported in elvish and zsh.
Since notifications are persistent in elvish it is only shown when there are no values to complete.

Tags
Completion values can now be tagged. Based on this they are grouped in zsh.

Command groups
Subcommands can now be grouped and will be highlighted in different colors.

Group names
Group names can be shown in zsh with the following zstyle.
zstyle ':completion:*' format $'\e[2;37mCompleting %d\e[m'
Group ordering
Order of the groups can be configured in zsh with the following zstyle.
zstyle ':completion:*:git:*' group-order 'main commands' 'alias commands' 'external commands'
_describe
_describe is now used in zsh which enables the grouping of tags.
With this aliased commands and flags are now also arranged side by side.
