Package extensions

[aplavin]

This package defines rules for functions from LogExpFunctions, NaNMath, and SpecialFunctions packages, in addition to Base. Looking at https://github.com/JuliaDiff/DiffRules.jl/blob/master/src/rules.jl, their rules are pretty self-contained (*) and can easily be turned into package extensions to enable using DiffRules as a dependency in other lightweight packages.
Would you support and accept such a PR turning them into weakdeps?

(*) One exception: a SpecialFunctions function used in a NaNMath rule definition,

DiffRules.jl/src/rules.jl

Line 276 in 8842177

@define_diffrule NaNMath.lgamma(x) = :( SpecialFunctions.digamma($x) )

.

[devmotion]

My impression from #90 was that the current design doesn't allow to define rules in extensions.