Gracefully handle missing targets

Aurget often finds a PKGBUILD for a package not actually in the AUR.
This can cause some confusing errors for dependency packages. These
cases typically indicate a package that has moved to the official
repositories but hasn't had its PKGBUILD cleaned off the AUR servers
yet.

In the case of a dependency, we should just skip it. Makepkg will either
find it or error more informatively itself. For normal arguments, we
continue to error; it's still invalid usage.
Update Known Bugs in docs
Ensure --nocolor is respected right away

We need to look for the option early, setup_colors once before sourcing
config (because it logs), and again after (because color settings may
have changed).
Doc update

Remove Known Bug (fixed), correct grammatical error.
Remove unused Makefile var, version bump
Fix kramdown-usage in Makefile, render manpage
Releasing 4.0.13-1