* Add pip completion
* We call native pip completion for fish if pip is installed
* Add pipenv completion
* We call pipenv native completion if pipenv is installed
* Applied changes as requested by @floam
* Changed usage of `test (command -v)` for just `command -sq`
* Add completions for pip2/3
* In some systems pip is not aliased and we have pip2 and pip3
* In those cases, we just load the completions for those commands
* Separate pip2/3 completions in their own file as requested by @floam
