Verbose mode

[kylekyle]

Add a verbose flag that shows the user all of the output from the installation. If verbose isn't requested, just print "Dependencies successfully installed".

[somedave]

Wanted to bump this one again @gershwinlabs It would be nice if the default output of ipydeps could mirror iruby/dependencies with a red "Installing dependencies" banner replaced by a nice green "dependencies installed" banner once finished.

[billallen256]

Ok, capturing what irdeps does for my future self:

https://github.com/nbgallery/iruby-dependencies/blob/master/lib/iruby/dependencies.rb#L66