Add implicit multiplication

[cshaa]

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Make this:

 a^2 + 2 a b + b^2

parse as this:

  a**2 + 2*a*b + b**2

The implicit multiplication operator can be overloaded (see #30) using options.operators.juxtapos. If you want to disable multiplication by juxtaposition, redefine the operator to throw an error.

[cshaa]

I couldn't figure out the correct grammar for this, but if anybody wants to give it a shot, PRs are always welcome. However, I suspect it would be much easier to implement with a PEG instead of Jison (see #40).

[cshaa]

See this SO answer for a great analysis of the problem.