String Interpolation Support

[mattBrzezinski]

It would be nice if we could support string interpolation

foo = "foo"
bar = datadep"$foo"

ERROR: KeyError: key "\$foo" not found
Stacktrace:
 [1] getindex at ./dict.jl:467 [inlined]
 [2] resolve(::String, ::String, ::String) at /Users/mattbr/.julia/packages/DataDeps/gbyoa/src/resolution.jl:54
 [3] resolve(::String, ::String) at /Users/mattbr/.julia/packages/DataDeps/gbyoa/src/resolution.jl:73
 [4] top-level scope at REPL[10]:1

[oxinabox]

I would accept a PR that added String interpolation: that would be useful.
Technically could be breaing if someone was using $ in their datadep names, but i doubt anyone is since.

til then you can do @datadep_str foo to have this effect.