Show load and file_extensions in accessors

[oxinabox]

Like #26 this seemed an interesting intellectual exercise.
This functionality looks scary and technically it is internals.
But I have been using it in ExprTools.jl in like every version of julia back til 1.0
Having to use an internal method for this is another manifestation of JuliaLang/julia#20555

I put this in a helper function so that after #26 we could add it to that list also

[exaexa]

OK actually to keep people from getting confused, could we make the type_accessors a complete standalone "visible" method? It is a bit different from accessors (they don't "eat" an actual object).

EDIT: maybe even type_methods or so at that point, to make this even clearer.

Other than that this looks wunderbar, thanks again!

PS I see you rebased, perfect. :D

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@@            Coverage Diff            @@
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[oxinabox]

I think it is pretty clear from how it is printed:

julia> AbstractFBCModels.required_accessors()
[1] reactions(a::AbstractFBCModels.AbstractFBCModel) @ AbstractFBCModels none:0
[2] n_reactions(a::AbstractFBCModels.AbstractFBCModel) @ AbstractFBCModels none:0
[3] metabolites(a::AbstractFBCModels.AbstractFBCModel) @ AbstractFBCModels none:0
...
[11] save(a::AbstractFBCModels.AbstractFBCModel, path::String) @ AbstractFBCModels none:0
[12] load(::Type{A}, path::String) where A<:AbstractFBCModel @ AbstractFBCModels ~/repos/AbstractFBCModels.jl/src/io.jl:7
[13] filename_extensions(::Type{A}) where A<:AbstractFBCModel @ AbstractFBCModels ~/repos/AbstractFBCModels.jl/src/io.jl:25

but up to you.
Its getting late here so I will leave it with you, if that ok?
Feel free to push to this branch before you merge.

[exaexa]

actually yeah with the types it looks completely transparent. Thanks a lot!