continuous newlines -> single whitespace (#50)
merge two variables to g:openbrowser_format_message (#47)

Now the following variables are merged to g:openbrowser_format_message.
* g:openbrowser_show_message
* g:openbrowser_short_message

This commit also improves s:expand_keywords() function.
Now the function supports nested expression like
g:openbrowser_format_message default value:
    "opening '{uri}' ... {done ? 'done! ({command})' : ''}"
Merge branch 'format-message-variable'