(refs #15) Add a reader macro #`...` for executing a shell command easily.

CL21-USER> (use-package :cl21.process)
T
CL21-USER> #`ls -l /Users`
"total 0
drwxrwxrwt    5 root         wheel   170 Nov  1 18:00 Shared
drwxr-xr-x+ 174 nitro_idiot  staff  5916 Mar  5 20:47 nitro_idiot
"
""
0

CL21-USER>
Shell interaction is implemented in cl21.process now.