feat: manifest.json support

[fent]

Hi there,

I've added support for updating a manifest.json file, which is used in Google Chrome extensions.
There's potential in standard-release to be used for software packages that are not node.js related or don't use npm. And I think this is a good start towards that.

Thank you for this tool btw, it's helped me a lot!

[stevemao]

I guess we could merge this before we come up with a plugin strategy?

[fent]

Just noticed I left the .only on the test. But now some other tests fail.

Will try to figure out why.

[coveralls]

Coverage decreased (-0.1%) to 99.853% when pulling 4a030cd on fent:manifest into 9a99607 on conventional-changelog:master.

[coveralls]

Coverage remained the same at 100.0% when pulling afd1620 on fent:manifest into 9a99607 on conventional-changelog:master.

[coveralls]

Coverage decreased (-0.1%) to 99.853% when pulling 4a030cd on fent:manifest into 9a99607 on conventional-changelog:master.

[bcoe]

@fent I like this refactor.

[bcoe]

@fent please try, npm i standard-version@next it should have your changes.