Support for concatenation (Base.:*)

[XQP-Munich]

Currently, the concatenation of two LaTeXStrings using the operator * yields a String, not a LaTeXString.

I propose to overload Base.:*, to the effect of something like

function Base.:*(lhs::LaTeXString, rhs::LaTeXString)
	return LaTeXString(lhs.s[1:end-1]*rhs.s[2:end])
end

Note that this example is not a valid implementation, since LaTeXStrings do not need to start and end with $, for example when they contain text.

[stevengj]

Duplicate of #17.