Python's pip and pipenv completions (#4448)

* 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

share/completions/pip.fish

@@ -0,0 +1,4 @@
+if command -sq pip
+    eval (pip completion --fish)
+end
+

share/completions/pip2.fish

@@ -0,0 +1,3 @@
+if command -sq pip2
+    eval (pip2 completion --fish)
+end

share/completions/pip3.fish

@@ -0,0 +1,3 @@
+if command -sq pip3
+    eval (pip3 completion --fish)
+end

share/completions/pipenv.fish

@@ -0,0 +1,3 @@
+if command -sq pipenv
+    eval (pipenv --completion)
+end