Rename package: mgit2 => mrgit

[pomek]

Suggested merge commit message (convention)

Other: Renamed the package. mgit2 becomes mrgit. Closes #85.

BREAKING CHANGE: mgit2 is no longer supported. Install mrgit instead.

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Additional information

Before reviewing those changes, please review #105, #108 and merge if everything works and you don't have any issues.

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What to do after merging this PR:

1. Clear all changelog and releases.
2. Rename the repository s/mgit2/mrgit

or:

1. Close the repository.
2. Create a new repository (mrgit)

or:

1. Do not clear changelog/releases and release mrgit as 1.0.0.

Then:

1. Update CKEditor 5: Renamed and replaced usage: mgit => mrgit ckeditor/ckeditor5#1819
2. Update CKEditor 5 dev: Renamed and replaced usage: mgit => mrgit ckeditor/ckeditor5-dev#526
3. Check Travis Integration.
4. Check Coveralls Integration.
5. npm deprecate mgit2 "This package has been renamed and is not supported anymore. Install mrgit instead."

[coveralls]

Pull Request Test Coverage Report for Build 344

• 47 of 47 (100.0%) changed or added relevant lines in 12 files are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage remained the same at 99.251%

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Totals
Change from base Build 342:	0.0%
Covered Lines:	504
Relevant Lines:	504

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💛 - Coveralls

[pomek]

[Reinmar]

Finally, I dug deep enough in the PR queue to reach this one :D 🎉