use rvm-configuration-file-name and rvm-executable in octomacs-rake-with-rvm()
add octomacs-alternate-postdir as a customizable variable
add octomacs-rename-post() to use octomacs-alternate-postdir
call octomacs-rename-post() if octomacs-alternate-postdir is non-empty
correct typo in README
update README to document octomacs-alternate-postdir