Add pre-commit hook declaration

[adamchainz]

Fixes #429.

[shssoichiro]

Thanks! I appreciate this a lot, as my attention has been split between a lot of different projects lately.

[adamchainz]

👍

I have no idea why the tests are failing, it's not because of this change is it?

FYI I tested this on a repo with many (already optimized) PNG files like so:

$ pre-commit try-repo ~/Documents/Projects/oxipng --all-files
[INFO] Initializing environment for ../oxipng.
===============================================================================
Using config:
===============================================================================
repos:
-   repo: ../oxipng
    rev: 961c1548ec6daeed1edc841c0a41323ffa81e80d
    hooks:
    -   id: oxipng
===============================================================================
[INFO] Installing environment for ../oxipng.
[INFO] Once installed this environment will be reused.
[INFO] This may take a few minutes...
oxipng...................................................................Passed

[shssoichiro]

It looks like I need to bump the minimum Rust version, because one of the dependencies bumped theirs.

I'll do that, and then you can rebase and it should work.

[adamchainz]

Huzzah! Thank you. Tested and working great.

FYI to make this discoverable, I've submitted the hook to be displayed on the pre-commit site (pre-commit/pre-commit.com#617) and added a note to my mirror (https://github.com/adamchainz/pre-commit-oxipng#deprecated-2022-01-15).