feat: add maximum length flag (#91)

[ceIia]

adding the length flag in reference to #91.

i assumed the default value of the maximum length according to the 50/72 rule, but of course this is still an opinionated change and should be discussed according to what you guys think!

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i still have an issue though with commands/prepare-commits-msg-hook.ts, should it:

1. pass the length as an argument from cli.ts to generateCommitMessage,
2. hardcode the length inside the generateCommitMessage invocation,
3. make the length argument optional with a default value of 50 (or something else)
4. something better around the process.argv.slice line? (probably bad for maintainability)

thx for reading!

[vetan2]

I really want this feature, but sadly it has conflicts.

[privatenumber]

@ceIia Can you update the PR to resolve conflicts?

Also, can you make the maximum length a config property instead of a flag?

[ceIia]

sure!