Better text file handling

1. Turn on formatoptions 't' for text files
2. The text filetype doesn't work in some cases.  Add specific commands
   for files like README.
3. Map 'coa' to toggling of the formatoptions 'a', auto-format
   paragraphs.
4. Previous 'coa' -> 'cop', for toggling auto pairing (delimitMate).
5. Previous 'cop' -> 'co(', for toggling rainbow parenthesis.  This is a
   feature I've not used much, so maybe it needs to go away.

I'm debating the value of formatoptions 'n' for certain files.  It could
be helpful for some things but seems to break others.  For now it is not
being used.

With 'coa' present, there is some merit to considering adding 'a' by
default for text files, since it would then be trivial to turn off.  But
since 'a' autoformats paragraphs all the time, any 'hand' formatting
will be lost with the next key press in that paragraph.  So it's
probably better to leave it as a selectively enabled element as it is
now.