Suggest tmux if-shell instead of run-shell

Modify README to suggest use of if-shell (shortcut `if`) rather than
run-shell (shortcut `run`), this allows makes the configuration a bit
easier to read since it doesn't require as many levels of quoting and
has direct support for an "else" branch rather than requiring chained
boolean operators.

Also change from using `tmux display-message -p` to `echo` when
retrieving the current command.  The expansion is done by `run` or `if`
anyway, and echo should be lighter weight than running another tmux
process and is definitely shorter.