Add REVISION_SCRIPT_FILENAME token for post_write_hooks options

I include a token called `REVISION_SCRIPT_FILENAME` which can be used in the `options` setting of a post-write hook. This token is replaced by the filename of the revision script, allowing the filename to be passed to the post-write hook in a flexible manner. Thus I can do:

```ini
[post_write_hooks]
hooks = pre-commit
pre-commit.type = console_scripts
pre-commit.entrypoint = pre-commit
pre-commit.options = run --files REVISION_SCRIPT_FILENAME
```

Note that the existing behavior is to prepend `REVISION_SCRIPT_FILENAME` to the argument list. In order to preserve backwards compatibility, this is done when `REVISION_SCRIPT_FILENAME` is not present.

### Description
* Implement the `REVISION_SCRIPT_FILENAME` token.
* Switch from `str.split()` to `shlex.split()` for robust command line argument processing.
* Insert `REVISION_SCRIPT_FILENAME` in appropriate locations in the docs and templates.
* Properly document that `REVISION_SCRIPT_FILENAME` is prepended instead of appended to the argument list.
* Refactored existing "test of `black` as a post-write hook" in order to avoid code duplication.
* Add a test for `REVISION_SCRIPT_FILENAME` and shlex.

### Checklist
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This pull request is:

- [X] A new feature implementation
	- please include the issue number, and create an issue if none exists, which must
	  include a complete example of how the feature would look.
	- Please include: `Fixes: #<issue number>` in the commit message
	- please include tests.

Fixes issue #819

Closes: #820
Pull-request: https://github.com/sqlalchemy/alembic/pull/820
Pull-request-sha: 73dd0a4d60758a06551a0aff47b9723b0345d74a

Change-Id: Ibfc677d59086703872d7cfd5c2da32bd0220a25f

1 files changed, 4 insertions, 4 deletions

diff --git a/alembic/templates/generic/alembic.ini.mako b/alembic/templates/generic/alembic.ini.mako
index bf7e5d1..31de75a 100644
--- a/alembic/templates/generic/alembic.ini.mako
+++ b/alembic/templates/generic/alembic.ini.mako
@@ -48,10 +48,10 @@ sqlalchemy.url = driver://user:pass@localhost/dbname
 # detail and examples
 
 # format using "black" - use the console_scripts runner, against the "black" entrypoint
-# hooks=black
-# black.type=console_scripts
-# black.entrypoint=black
-# black.options=-l 79
+# hooks = black
+# black.type = console_scripts
+# black.entrypoint = black
+# black.options = -l 79 REVISION_SCRIPT_FILENAME
 
 # Logging configuration
 [loggers]