Add helper functions for $PATH manipulation (#90)

tests/autoenv_utils.t

@@ -0,0 +1,29 @@
+Tests for provided utils/helpers.
+
+  $ source $TESTDIR/setup.zsh || return 1
+
+  $ PATH=
+  $ autoenv_prepend_path custom_path
+  $ echo $PATH
+  custom_path
+
+  $ autoenv_prepend_path custom_path
+  $ echo $PATH
+  custom_path
+
+  $ autoenv_prepend_path another_path a_third_one
+  $ echo $PATH
+  a_third_one:another_path:custom_path
+
+  $ autoenv_remove_path another_path a_third_one
+  $ echo $PATH
+  custom_path
+
+  $ autoenv_remove_path does_not_exist
+  [1]
+  $ echo $PATH
+  custom_path
+
+  $ autoenv_remove_path custom_path
+  $ echo PATH:$PATH
+  PATH: