Add kwarg post

[gustaphe]

Adds a post kwarg to @latexdefine, which gives you a bit more control of how the final rhs is formatted.

julia> @latexdefine x=3.75 post=x->round(x; sigdigits=2)
L"$x = 3.75 = 3.8$"

julia> @latexdefine x=1//2 post=float
L"$x = \frac{1}{2} = 0.5$"

julia> using Unitful, UnitfulLatexify; m = 5u"kg"; v = 10u"m/s";

julia> @latexdefine E = m*v^2/2 post=u"J"
L"$E = \frac{m \cdot v^{2}}{2} = 250.0\;\mathrm{J}$"

I'm mostly interested in this last one for physics material, but I think this is general enough it could be useful in different contexts as well.

[codecov]

Codecov Report

Merging #220 (ffd834f) into master (1067732) will increase coverage by 0.60%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           master     #220      +/-   ##
==========================================
+ Coverage   63.24%   63.84%   +0.60%     
==========================================
  Files          23       23              
  Lines         827      838      +11     
==========================================
+ Hits          523      535      +12     
+ Misses        304      303       -1     

Impacted Files	Coverage Δ
src/macros.jl	100.00% <100.00%> (+7.69%)	⬆️
src/latexoperation.jl	84.89% <0.00%> (+0.10%)	⬆️

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