[FR] Add link to upstream on source file pages

[jan-glx]

rdrr-io works quite well and I often find the source I am looking for coming from google. Sometimes however I want to edit the found source, e.g. on github. Having a prominent link (perhaps even per line?) to the upstream repository would drastically simplify this.

For example,
https://rdrr.io/github/broadinstitute/inferCNV/src/R/inferCNV_heatmap.R
should link to https://github.com/broadinstitute/infercnv/blob/master/R/inferCNV_heatmap.R
or even better, line four there should link to https://github.com/broadinstitute/infercnv/blob/master/R/inferCNV_heatmap.R#L4