Always compile before running checks

[denisdefreyne]

(See nanoc/nanoc#1341)

It is currently possible to run nanoc check on a site that it out of date. To prevent nanoc check from running on a not-yet-compiled site, I suggest adding a compile option to the configuration’s check section, which by default is false, but if set to true, will run nanoc compile beforehand.

Additionally, a command-line flag (--compile/--no-compile) could be useful to override the configuration behavior.