integrate g:openbrowser_open_commands,g:openbrowser_open_rules into g:openbrowser_browser_commands
fix syntax error: missing comma
implement backward compatibility for g:openbrowser_open_{commands,rules}
change structure of g:openbrowser_browser_commands
ready for a merge into master
ready for a merge into master

I gave up 'git rebase -i'...
Merge branch 'option-browser-commands'
fix doc for g:openbrowser_browser_commands
fix: main code in s:open_browser() was incorrectly reverted at d380049