Latexify as soft dependency

[gustaphe]

I wanted to lift the idea of merging UnitfulLatexify as an extension, now that those are possible. This will remove some user level friction, using Unitful, Latexify will just work.

Why here and not Latexify? It's an arbitrary choice, but how a unit is latexified is a property of the unit. For instance, UnitfulLatexify contains a table of exceptions, which seems more likely to update due to a version change in Unitful than one in Latexify.

I can make a PR if you agree this is a good idea.

https://github.com/gustaphe/UnitfulLatexify.jl

[giordano]

Sounds good