Pipenv: consider locking after every install

[kennethreitz42]

[jpadilla]

+1 on this. npm updates its shrinkwrap.json if installing/saving the package.json and yarn does the same with yarn.lock.

I think it might be a good rule that if we're updating the Pipfile, we also update the Pipfile.lock, instead of having the user do so everytime. I could tackle this.

[kennethreitz42]

the only downside is that currently locking is a rather expensive (time-wise) operation.

[rmad17]

How about an optional parameter to check for locking after install?

[kennethreitz42]

@rmad17 done!