README.md: escape "bare" underscores
README.md: escape "bare" underscores

Even though some Markdown interpreters do not give special meaning
to these particular underscores (because they were inside
asterisks?), they should be escaped so that some other Markdown
interpreter (e.g. redcarpet in its "native" mode) will not interpret
them as emphasis/strong delimiters.

Tested with rdiscount, redcarpet and RedcarpetCompat.

This problem did NOT show up when GitHub renders README.md (they use
RedCarpetCompat?), but it is probably good to fix it anyway since it
does confuse the default redcarpet.

bradparks/tmux-MacOSX-pasteboard,ChrisJohnsen/tmux-MacOSX-pasteboard,tjt263/tmux-MacOSX-pasteboard,lembacon/tmux-MacOSX-pasteboard