articles/nanobsd: Add section on customizing NanoBSD

nanobsd.sh can be passed several options, and having this documented is
a good idea, as it even includes the ability to customize the
third-party software on the NanoBSD image.

While here, fix up some uses of applications tags, and some incorrect
capitalization.

Submitted by: Yoan Adida <yoanadida at gmail.com> (previous version)
Reviewed by: bcr, imp, wblock (some of an earlier version),
Mina Galić <me@igalic.co>, PauAmma
Differential Revision: https://reviews.freebsd.org/D7406