form vendor link to external repos for flowchart.js and raphael.js

[Сергей Роганов]

Make vendor link to external repos for flowchart.js and raphael.js at this repo to have actual version for each update. (see https://github.com/oohoo/moodle-filter_mathjax/tree/master/vendor) Save local downloaded versions in archive for safety.

[Сергей Роганов]

see git submodules:

http://blogs.atlassian.com/2011/12/git-submodules/

http://blogs.atlassian.com/2013/05/alternatives-to-git-submodule-git-subtree/

http://git-scm.com/book/en/v2/Git-Tools-Submodules