Make auto-workon work with text files containing virtualenv name

This way you can keep all your virtualenvs in one place and can reuse
the same virtualenv for multiple projects.

Virtualfish (virtualenvwrapper for fish shell) does this as well,
see: http://virtualfish.readthedocs.io/en/latest/plugins.html#auto-activation-auto-activation

README.md

@@ -324,10 +324,11 @@ instead:
          (projectile-find-file)))
 ```
 
-As long as a virtualenv is found in the `projectile-project-root` and
-whose name is in the list `venv-dirlookup-names` it will be
-automatically activated. By default, it's value is `'(".venv", "venv")'`,
-but you can set if however you like to match your naming conventions:
+As long as a virtualenv or a text file with the name of the virtualenv is
+found in the `projectile-project-root` and whose name is in the list
+`venv-dirlookup-names` it will be automatically activated. By default, it's
+value is `'(".venv", "venv")'`, but you can set if however you like to match
+your naming conventions:
 
 ```lisp
 (setq venv-dirlookup-names '(".venv" "pyenv" ".virtual"))

virtualenvwrapper.el

@@ -94,12 +94,19 @@ to activate when one of them is found."
   "If a venv in the projetile root exists, activates it.
 Set your common venvs names in `venv-dirlookup-names'"
   (let ((path (--first
-                (file-exists-p it)
-                (--map (concat (projectile-project-root) it)
-                        venv-dirlookup-names))))
+               (file-exists-p it)
+               (--map (concat (projectile-project-root) it)
+                      venv-dirlookup-names))))
     (when path
-      (setq venv-current-name path) ;; there's really nothing that feels good to do here ;_;
-      (venv--activate-dir path))))
+      ;; If the PATH is a regular and readable file, read the first
+      ;; string in this file and use it to active the virtualenv.
+      (if (and path (file-regular-p path) (file-readable-p path))
+          (with-temp-buffer
+            (insert-file-contents path)
+            (venv-workon (car (split-string (buffer-string) "\n" t))))
+        ;; PATH is not a file, assume it's a virtualenv directory and activate it
+        (setq venv-current-name path) ;; there's really nothing that feels good to do here ;_;
+        (venv--activate-dir path)))))
 
 
 ;; internal utility functions