Support enum values with prefix

[zbiljic]

The change adds support option that trims enum name prefix from enum values.

This is for the case when the proto file is written according to Google Protobuf Style Guide recommendation. This suggest prefixing enum values instead of surrounding them in an enclosing message.
Example:

enum FooBar {
  FOO_BAR_UNSPECIFIED = 0;
  FOO_BAR_FIRST_VALUE = 1;
  FOO_BAR_SECOND_VALUE = 2;
}

When this new option is enabled, it will produce something like this:

"enum": [
    "UNSPECIFIED",
    "FIRST_VALUE",
    "SECOND_VALUE"
],

NOTE: There is one additional commit which is related to imports. The make fmt command changes existing imports, but I decided on a separate commit because it would seem it was not run before (maybe for some reason).

Also, when running make fmt on existing code, it changes one additional file, generated options.pb.go, which should not be modified, but it seems like goimport tool does not ignore generated files. I would suggest updating command call to something like this:

@goimports -w -local github.com/chrusty/protoc-gen-jsonschema $(shell find . -type f -name '*.go' -not -path "./internal/protos/*")

P.S.
Sorry for not opening an issue first. I figured this was small change and would be fine to just create pull request for.

[chrusty]

Hi @zbiljic, and thanks for this PR! I appreciate the effort you've made to fit in with this weird framework i've got here of flags, options, and unit tests.

The change looks good to me, and I'll be happy to merge this in - I would just request a few extra comments around where the logic is injected in converter.go (it will make it easier for people working on this file in the future).

[chrusty]

Thank you @zbiljic - nice PR. I've merged this and made a new release.

Thanks for your interest in the project too!

[zbiljic]

Thanks for this, and this project.

I do have plans to use it actively, and might have some additional changes soon.