Add --tag digest to commands that work with images

Changelog for 0.16.9:

• PR #967 Generate SHA digest for image tags for faas-cli commands by @alexellis
• PR #962 add new watch flag for up command by @NikhilSharmaWe

Commits
cfbfce6 Add --tag digest flag to faas-cli generate by @alexellis
3af5f4d Initial version of generating SHA from function source by @alexellis
a982889 add new watch flag for up command by @NikhilSharmaWe

Changes: 0.16.8...0.16.9

Generated by Derek