ghi open: Support multiple occurences of "-m" option to set title and description comment in one go like "git commit" does

The command "git commit" features the "-m" option like ghi but allows multiple occurences. The help of "git commit" says:

   -m <msg>, --message=<msg>
       Use the given <msg> as the commit message.
       If multiple -m options are given, their values are
       concatenated as separate paragraphs.

Currently, ghi takes a second -m option which overwrites the first argument, i.e. re-sets the title. Adding text with newlines as argument to -m does split into title and description already, like this

$ ghi open -m "test issue4
> this should go into description"

but this is not a very convenient style and unusual. Also, the help message does not tell about it. I did take a look into the source code to see this :-)