Allow foldmarker=[[[,]]] and formatoptions; add help doc/securemodelines.txt, add README

[step-]

The starting text for README.md and securemodelines.txt came from the vim.org page for this plugin. I rearranged and updated a few paragraphs. Then I added option 'formatoptions' for reasons explained in the commit comment.

I added support for foldmarker, but limited to marker [[[,]]]]. The reason is that the characters that consitute the foldmarker need to be added to the regex in DoOne. Since I use [[[,]]] I added [ and ] to the regex. Square brackets can be useful for other people too as an alternative to curly braces {{{,}}}

    vim: fmr=[[[,]]]:

I would also like to add support for iskeyword, but haven't gotten around it. Motivating example for supporting iskeyword is /usr/share/vim/vim74/doc/help.txt

    vim:tw=78:fo=tcq2:isk=!-~,^*,^\|,^\":ts=8:ft=help:norl:

[ches]

Bravo for the help doc.

[step-]

Thank you.