Add govendor as dependency manager

[gtsalles]

What do you think about using govendor as default dependency manager for the project?

Right now, the only external dependency existing is justext, but in #26 I've added a new dependency to deal with xlsx files, in the future, new dependencies may be added for dealing with new file types, and this could be a good approach for handling the dependency management.

[mish15]

We're likely to replace justext soon, so likely no external Go deps