Enhancements to REPL commands

[jpellegrini]

• Mention that ls accepts usual arguments
• Make ,o use the default-browser procedure (otherwise, at least on my Linux system, it won't work, although I can't see exactly why)

[egallesio]

Thanks @jpellegrini for this PR. I have adapted it a bit:

• shorten the message for ls to fit in the 80-character limit when using help
• added the ,b which calls the default browser and keep the ,o command as is (since they are not the same). For instance, using ,o foo.stk will launch emacs on file foo.stk (if xdg-open is configured), or o . will open the file browser in the current directory. On the other hand, ,b foo.stk will probably launch emacs, but after having opening your browser.