Allow fractional inputs to thread count flag

[LvlAndFarm]

Adds support for fractional thread counts when using thread count flag -j

E.g.:

-j 1 => 1 thread
-j 3 => 3 threads
-j 0.75C => 3 threads on a 4-core machine
-j 1C => 4 threads on a 4-core machine

Closes #3482

[LvlAndFarm]

@lihaoyi I've changed the PR now to take fractional thread counts with the C prefix e.g. -j 0.75C

[lihaoyi]

Will take a look tomorrow! It's bedtime here now

[lihaoyi]

@LvlAndFarm I'm picking #3554 for the bounty, somewhat arbitrarily, but I'm going to close this one. Thanks for the time you put in, and feel free to look at the other bounties if you're still interested at trying your hand :)