Fix processing of multiline expressions

[kenjknox]

Wanted to allow the processing of block expressions like:

quote a = 1
    b = 2
    c = 3
end

However the current behaviour of latexify is only to return the last line of the expression.

The external fix is to make a custom function to strip the LineNumberNodes and return as an array:

latexifyquote = (ex::Expr; kwargs...) -> latexify(filter(x->!(x isa LineNumberNode), ex.args); kwargs...)

However I expect this is functionality which should belong in the base package, since it is fairly useful and doesn't (to my knowledge) conflict with any other feature.

The core change is adding a function to check if any argument is a Expr containing LineNumberNode and then splitting it into an array so all lines can be processed correctly. Finding the LineNumberNode is assumed to indicate a multiline Expr only.