Allow extensions to be specified on command line

[sammarshallou]

Adds an 'extensions' option to config which takes a comma-separated
list of extensions to add to the default set. Then adds --extensions
parameter to each command-line tool to specify this value.

This allows use of MathJax features such as displaying invalid TeX
instead of failing to render (--extensions TeX/noUndefined), and
stripping out dangerous links from MathML (--extensions Safe).

[sammarshallou]

This follows on from #72. Here are some examples of usage with the 'noUndefined' TeX extension:

$ bin/tex2svg 'a^\frog'
TeX parse error: Undefined control sequence \frog

$ bin/tex2svg 'a^\frog' --extensions=TeX/noUndefined
<svg xmlns:xlink="http://www.w3.org/1999/xlink" ...

Or with the 'Safe' extension, for example with MathML:

$ bin/mml2svg '<math><mtext href="javascript:alert()">X</mtext></math>'
...
<a class="mjx-svg-href" href="javascript:alert()">
...

$ bin/mml2svg '<math><mtext href="javascript:alert()">X</mtext></math>' --extensions Safe
... (SVG code with no link) ...

[dpvc]

This could be simplified as

window.MathJax.extensions.push(name+'.js');

[dpvc]

This looks good, but I did make a couple of comments in the code.

[sammarshallou]

Thanks for comments! I've amended my commit to include the 3 requested changes:

1. array.push (don't understand why I wasn't doing that in the first place)
2. I hadn't realised you could specify the extensions at top level, that does make the code an awful lot simpler!
3. I made it allow .js

I retested it only using the examples I gave above (except I also tried adding .js this time), which is a bit less thorough than the testing I did last time but probably okay. Seems to still work...