Add support for \cancel and co.

[Orace]

This commit address #349 and add support to:

• \cancel
• \bcancel
• \xcancel
• \cancelto

Current output of \cancel{Q} \\ \xcancel{69} \\ \frac{a\bcancel{b}}{\bcancel{b}}:

[ForNeVeR]

Great work! There's a couple of minor issues left, could you please take a look?

I should note that this is a breaking change technically, but a good one and for a good reason, so I approve that.

[Orace]

@ForNeVeR, this is a draft work, I'm open to (and ask for) any advice on this.

[Orace]

\cancelto need an arrow, I don't know how to draw this, maybe something like what is done for the radical atom (sqrt, √), maybe we should work on this in another PR (I will be AFK for several weeks soon).

[ForNeVeR]

Thanks!

[ForNeVeR]

I think we'll leave \cancelto still open for grabs for now.