allow specifying custom build tags

[vikstrous2]

wire relies on custom build tags in initializer files to generate code. The files that are loaded/built by default are part of the generated output and the files that rf should really be refactoring are not built unless the wireinject built tag is set. There should be a way to tell rf to use this build tag when loading and refactoring code.

There may be a larger question of how to refactor every version of code that relies heavily on build tags, but I don't want to open that can of worms. I hope it's simple enough to specify a single set of tags to use when loading/refactoring code.

[vikstrous2]

I was able to specify custom build flags by setting GOFLAGS before running rf:

export GOFLAGS='-tags=wireinject'

Maybe that's good enough. Feel free to close this issue.