Expose field mask paths and remove obsolete method

[johanstokking]

Summary:

Closes #12

Changes:

• Expose top level and nested field mask paths
• Remove method that copies and returns them

[rvolosatovs]

Please fix tests and regenerate the golden files (make test)

[johanstokking]

For the record, make test doesn't work for me;

Regenerating golden files...
Running tests...
--- FAIL: TestGolden (1.58s)
    main_test.go:57: Running 'docker run --user 501 --rm --mount type=bind,src=/Users/johan/dev/go/src/github.com/TheThingsIndustries/protoc-gen-fieldmask,dst=/Users/johan/dev/go/src/github.com/TheThingsIndustries/protoc-gen-fieldmask -e IN_TEST -w /Users/johan/dev/go/src/github.com/TheThingsIndustries/protoc-gen-fieldmask thethingsindustries/protoc:3.0.17 --plugin=protoc-gen-fieldmask=/Users/johan/dev/go/src/github.com/TheThingsIndustries/protoc-gen-fieldmask/.work/tmp.qCm/go-build302900878/b001/protoc-gen-fieldmask.test -Ivendor -Itestdata --fieldmask_out=/Users/johan/dev/go/src/github.com/TheThingsIndustries/protoc-gen-fieldmask/.work/tmp.qCm/fieldmask-test819603683 --gogo_out=/Users/johan/dev/go/src/github.com/TheThingsIndustries/protoc-gen-fieldmask/.work/tmp.qCm/fieldmask-test819603683 testdata/testdata.proto'...
    main_test.go:60: Output:
        /Users/johan/dev/go/src/github.com/TheThingsIndustries/protoc-gen-fieldmask/.work/tmp.qCm/go-build302900878/b001/protoc-gen-fieldmask.test: program not found or is not executable
        --fieldmask_out: protoc-gen-fieldmask: Plugin failed with status code 1.
    main_test.go:66: Error: exit status 1
FAIL
exit status 1
FAIL	github.com/TheThingsIndustries/protoc-gen-fieldmask	1.589s
make: *** [test] Error 1

I updated to 3.0.17 because the internet is too shit here to pull 3.0.15. If I'm doing something wrong, i.e. if thethingsindustries/protoc needs to be rebuilt (first), please document that.

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