Add macro for latexifying one-liner functions and add external rendering.

[korsbo]

Poor practice, but this PR contains two separate new features.

One is the exported function render which tries to render a LaTeXString using lualatex and open the result in an external viewer. Useful for rendering things when in the REPL.

The other is a macro @latexify which can be used to directly latexify one-liner Julia functions.

julia> @latexify f(x, y=2) = x^2 + y
$\mathrm{f}\left( x, y = 2 \right) = x^{2} + y$

This macro call will also define f as expected.

No documentation or test yet.

[coveralls]

Coverage decreased (-2.3%) to 76.134% when pulling af5cf56 on render into fb34189 on master.

[codecov]

Codecov Report

Merging #100 into master will decrease coverage by 2.20%.
The diff coverage is 30.76%.

@@            Coverage Diff             @@
##           master     #100      +/-   ##
==========================================
- Coverage   79.29%   77.09%   -2.21%     
==========================================
  Files          22       23       +1     
  Lines         512      537      +25     
==========================================
+ Hits          406      414       +8     
- Misses        106      123      +17     

Impacted Files	Coverage Δ
src/Latexify.jl	75.00% <ø> (ø)
src/utils.jl	0.00% <0.00%> (ø)
src/latexoperation.jl	92.77% <100.00%> (+1.74%)	⬆️
src/macros.jl	100.00% <100.00%> (ø)

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[korsbo]

I disabled execution from @latexify. This macro now just treats the argument as an expression for latexification, without executing that expression.

The new macro @latexrun will first execute the expression and then latexify it.

Both should work for some simple maths-looking functions like f(x; y=1) = x + y.

[korsbo]

Although the approach is different than I imagined, I would say that this PR closes #28.