Execute a single drush command on multiple sites

It should be possible to execute a single drush command and have it execute on multiple sites, either local, remote, or a combination of local and remote sites. This should be done either by supplying a comma-separated list of site specifications or site aliases wherever site aliases are accepted, or by using a special site alias that represents a collection of sites.

For example:

drush dev.site1.com,dev.site2.com,dev.site3.com updatedb

or

drush all-dev updatedb

In the case of the second example, the 'all-dev' group alias is declared as follows:

A site alias is a group alias if it has a 'group' attribute but no 'uri' attribute. It is an error for an alias to have both 'group' and 'uri' attributes. A group may be made of a list of site specifications or site aliases.

It is also possible to use a group or group alias as the target of an sql sync command, like so:

drush sql sync live dev.site1.com,dev.site2.com,dev.site3.com

or

drush sql sync live all-dev

It is not possible to use a group alias as the source of an sql sync command or as the source of a sync command. The following might be supported:

drush sync \!drush all-dev:\!drush

However, it will not be possible to use group lists in the target of drush sync.

I will post a patch here when one is ready.