Add more advanced commands.
Change to GNU manual descriptions.

Mac OS X (BSD) manual descriptions and 'builtin' were used.
Add history.

Detail basic use of history command in conjunction with the arrow-keys
and the bang syntax.
Add description of which.
Fix subsection to subsubsection.
Add description of grep.
Fix typo and use better word.

pipping -> piping
insensitive to -> ignoring
Add description of sed, with mention to awk.
Add description of find and xargs.
Add description of ln.
Add descriptions: jobs, bg, fg, ps, kill, killall.
Remove ambiguities.

+ Describe meaning of builtin.

+ Restructure rm -rf description to be more clear.
Merge branch 'feature/adv' into feature/terminal

Merge in the more advanced program and command subsections into the
terminal section.