Load `datomic.clj` from `.lein/profiles.clj`

- but only if it exists

I'm not sure this is what I'll do long-term, but I don't like the
alternative (GPG-encrypted credentials), because it still doesn't seem
to solve the "Don't add the Datomic repo to every project" problem.
Rewrite `xterm-colortest` `--ramp` and `--num`

Not 100% happy with the `grid` function, but overall this is waaay
better than the horrible script I used to have.
Export `MAKEFLAGS` for parallel compilation

- `nproc` is a standard Linux command that does (exactly?) what my
  homegrown `cpus` command was doing
- Keeps separate `$pmake` parameter (for things that unset `$MAKEFLAGS`)
- uses `N` instead of `N+1` (`N+1` made more sense with `distcc`)