Support quoted strings

[vlopezj]

The last ASCIIMath version supports using quotes to delimit text elements, like so:

Input:

"time" = "money"

Result:

<math xmlns="http://www.w3.org/1998/Math/MathML">
  <mstyle displaystyle="true">
    <mrow>
      <mtext>time</mtext>
    </mrow>
    <mo>=</mo>
    <mrow>
      <mtext>money</mtext>
    </mrow>
  </mstyle>
</math>

This commit brings the behaviour to the python implementation.

[ShadowKyogre]

Heya @vlopezj . I just applied this over here - https://github.com/ShadowKyogre/python-asciimathml/tree/lang-2.2-port .

I'm also working on the other parts of the asciimathml 2.2 syntax atm.