Add ** include: directive for external file inclusion

[stormi]

This implements #25

---

Add ** include: directive for external file inclusion

Add support for including another test file at a point in the current
test file with ** include: file.t

The included file is parsed recursively — its tests, directives, and
nested includes take effect as if pasted inline at the ** include:
line. Paths are resolved relative to the including file's directory.

Multiple ** include: lines are needed to include several files; each
line includes exactly one file.

- Circular include chains are detected and raise an error with the
  full cycle path (e.g. A -> B -> C -> A)
- Duplicate test names across files raise an error showing both
  source files
- All directives follow the same rules: ** game: overrides,
  ** precommand: appends, etc.

---

I have tested this on my own tests, with snippets that advance the game from one state to another, and others that move you to a given state from the beginning of the game.

$ ls tests/includes/
gue_to_parvis  parvis_to_chambre  to_chambre  to_parvis

Here's one that moves you from "parvis" to "chambre" :

$ cat tests/includes/parvis_to_chambre 
* _parvis_to_chambre
> prendre corde et bois
> attacher corde et bois
> jeter grappin sur herse
> tirer corde
> pousser la charrette
> monter sur la charrette
> monter
Dans la chambre

And another that moves you from the beginning of the game to "chambre", by including other snippets:

$ cat tests/includes/to_chambre 
** include: to_parvis
** include: parvis_to_chambre

* _to_chambre
>{include} _to_parvis
>{include} _parvis_to_chambre

In the real test files themselves, I include the file snippets that I need and either {include} the "tests" they define via ** precommand or directly in the tests, depending on the case.

Example (WIP test file):

$ cat tests/walkthrough.test
** include: includes/gue_to_parvis
** include: includes/parvis_to_chambre

* walkthrough direct
>{include}_gue_to_parvis
>{include}_parvis_to_chambre

> o
> n
Dans le cabinet de l'abbé
[...]

[stormi]

test_include.py

Here's a test file that I haven't committed because the plotex project has no test framework at the moment, but that can be used to verify that the implementation does what it claims it does.

[stormi]

Last force-push: fixed stale commit message.