factor out and improve common code in vimpager/vimcat

[rkitover]

As I was working on vimcat a couple days ago, I was thinking there are some things that both utilities do that we can factor out into inc/.

[lucc]

Or we could merge both scripts into one.

Another idea which I tried a while back was to split vimcat into a shellscript and a vim script (like vimpager). The rationale behind this was that vimcat is either run from the repo, installed or bundled with the standalone vimpager, In the first two cases it is obvious that it need not be a mixed source script. And when it is extracted from vimpager it could just as well be extracted as a separate shell script and vim script. I will send a PR and you can look at the code. That will explain it best. (Need to rebase first and makefile changes/bundling is still missing)

[lucc]

Part of this is done in #196.