change g:openbrowser_format_message to Dictionary type
implement g:openbrowser_format_message.truncate
MIN_URI_STR_NUM should be in g:openbrowser_format_message
remove protocol in URI at first
doc: update g:openbrowser_format_message
-3 for "..."

s:Prelude.truncate_skipping() assumes also the 4th arg
is included in 2nd arg length.
Merge branch 'truncate-output-message' (close #54)