remove cgo dependency to allow for cross-compilation

[yashbhutwala]

[kroppt]

A workaround for this is to use v0.6.0, which does not depend on cgo. However, you will probably still need graphviz.

[ondrajz]

Suggestion from @kroppt can work as workaround, but I think this should be solved by supporting non-cgo builds and making the cgo optional.

This could be done by providing two implementations for the graphviz rendering, one that uses cgo library and contains // +build cgo and the other as a fallback using the system graphviz tool for rendering graph and contains // +build !cgo.