Add cwd post-write hook option

Add cwd post-write hook option

Fixes #822

Implements a `.cwd` suboption for post-write hooks.

For example, one 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
pre-commit.cwd = %(here)s
```

This feature is illustrated by the last line above.

### Description

* Improve the names of some variables recently created in #820 in order to make the code more readable.
* Add `cwd` as an option which is passed directly into `subprocess.run()`
* Add a brief description to the documentation, together with an example with `pre-commit`.
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### Checklist
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This pull request is:

A new feature implementation

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

**Help requested:** I can't think of any simple way to write a test. Does anyone have a suggestion???

Closes: #823
Pull-request: https://github.com/sqlalchemy/alembic/pull/823
Pull-request-sha: 9a694a7fdfc55160d1e124f6119a7a5677ce019a

Change-Id: Iacbb06d52acc362588cff2cdfec442393be8594a