adding "fig pull [SERVICE]" to pull service images

Fixes #158

Signed-off-by: Luke Amdor <luke.amdor@gmail.com>
adding 'fig pull' to cli docs

Signed-off-by: Luke Amdor <luke.amdor@gmail.com>
Merge pull request #411 from Banno/fig-pull

adding "fig pull [SERVICE]" to pull service images