CONTRIBUTING.md: formatting fixes
Use builtins when possible

a call to cat can be replaced by a redirection.
Use less wc calls

Use grep -c to avoid a call to wc
Use a redirection to avoid a call to cut
Use a redirection to avoid a call to cut (second)
Use $() instead of ticks
Consistent style for loops (while and for)
Support for counting screen sessions running with extra parameters

In some cases (multi user enabled in screen for example), the count of
detached screen sessions was incorrect. The command to locate these has
been improved using a more tolerant regex, which does not require
'detached' to be the first word inside the parenthesis.

To test, open a new screen session, add multiuser (type Ctrl+A, then
':', then multiuser on), then detach (Ctrl+A, Ctrl+D), and see that
liquidprompt detects the screen, as would "screen -list".
CONTRIBUTING: more policies