Added the logic to write rlx(1) shell commands to ~/.rlx/.history
including the logic to read the history file when showing the command
prompt so that history is maintained between multiple shell invocations.
Experimental work on adding tab completion for rlx(1) shell commands.
Ongoing work on the command completion logic for rlx(1) including adding
HISTSIZE and HISTFILESIZE to prevent slowdown on huge history files.