Add go fmt target to Makefile

[lanzafame]

I have added the fmt target as a dependency of the test target as this suits a local development workflow well (i.e. add/change code -> make test => go fmt && go test) but I am not sure if this is ideal as that would mean that the Travis CI build will also run go fmt, but I will defer the decision to the maintainers.

The fmt target affects the fixtures directory and does change the formatting of the fixture files. I am assuming by the nature of the project (AST -> JSON) that the formatting shouldn't matter but thought it worth noting.

[codecov-io]

Current coverage is 55.24% (diff: 100%)

Merging #39 into master will not change coverage

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[patrickt]

I use Emacs to hit gofmt automatically, but this looks good to me! Thank you!