add echo action

[rolandwalker]

Untestable since save-display ignores the status area.

Usecase:

bind blame   O  @open '%(file)'
bind blob    O  @open '%(file)'
bind diff    O  @open '%(file)'
bind grep    O  @open '%(file)'
bind stage   O  @open '%(file)'
bind status  O  @open '%(file)'
bind tree    O  @open '%(file)'
bind help    O  :echo 'Nothing to open'
bind log     O  :echo 'Nothing to open'
bind main    O  :echo 'Nothing to open'
bind pager   O  :echo 'Nothing to open'
bind refs    O  :echo 'Nothing to open'
bind stash   O  :echo 'Nothing to open'

[jonas]

Makes sense. Thanks.