Proposal: IEx.Helpers.use_if_available/2

[Wojtek Mach]

Hello,

I'd like to propose to introduce a `use_if_available/2` to complement `import_if_available/2`.

On our project we can do `use Shared` to get common aliases and imports. We can't use it in `.iex.exs` though because it won't work without `-S mix`; we have to copy-paste stuff to .iex.exs.

Similar small inconvenience on hexpm: [1] [2]

It's obviously not the end of the world to do some copy-pasting. I'd be happy to prepare a PR if accepted!

[1] https://github.com/hexpm/hexpm/blob/ce72c361e6374c28a81a2d4413677bc97294b1e2/.iex.exs#L2:L36

[2] https://github.com/hexpm/hexpm/blob/ce72c361e6374c28a81a2d4413677bc97294b1e2/lib/hexpm/web/web.ex#L98:L137

[José Valim]

Please do send a PR!

--

José Valim

www.plataformatec.com.br

Founder and 

Director of R&D