Make updating easier

[grimnight]

It should be easier to tell when a venv needs to be updated and to update.

Add outdated <venv> command to show outdated packages like Homebrews brew outdated and updates does.
Add upgrade --all option to upgrade all venvs.

[untitaker]

Should this upgrade dependencies as well or just the main package? I am also not sure if we can query pip for outdated packages efficiently.

[RonnyPfannschmidt]

needs enhancements on the pip side

[RonnyPfannschmidt]

i created pypa/pip#2847 and pypa/pip#2847

[RonnyPfannschmidt]

This one is also depending on on #52

[RonnyPfannschmidt]

Given pip7 recreating the env might work quicker

[anlutro]

I'm just doing pipsi list | grep 'Package ' | cut -d\" -f2 | xargs -n1 pipsi upgrade to upgrade all my pipsi installed packages. Would there be anything wrong with letting pipsi upgrade --all do the same sort of thing?