Add support for an --all-shells flag for ensurepath

[gh271]

How would this feature be useful?

I have an install script containing pipx ensurepath. This script has a #!/bin/bash shebang, meaning pipx is always added to $PATH in Bash. However, if someone using a shell other than Bash runs this script, the path is still only added to Bash. MacOS for example uses zsh by default.

An --all-shells flag for pipx ensurepath would be useful in my install script since pipx could then be added to all shells' $PATH, regardless of which the user uses.

I came across the following issue: #1290, which would be solved by an --all-shells flag I think.

Describe the solution you'd like

pipx uses the prepend() and append() functions from the userpath library. Both of these functions support an all_shells argument, which when set to True, sets the path for a list of pre-defined shells (https://github.com/ofek/userpath/blob/master/userpath/shells.py). This means it should be very easy to add an --all-shells flag.

I will create a PR for this. But I would obviously like to hear if you think this is a good idea!

Describe alternatives you've considered

I have not considered any alternative.