Add some completion for Bundler

Tested-by: Mark Somerville <marks@1partcarbon.co.uk> [Arch, MRI 2.0.0]
Signed-off-by: Mark Somerville <mark@jellyjelly.net>
Add command completion for gem

Tested-by: Mark Somerville <marks@1partcarbon.co.uk> [Arch, MRI 2.0.0]
Signed-off-by: Mark Somerville <mark@jellyjelly.net>