Allow indexing FileTree with string keys with path components in them

[mbauman]

Currently:

julia> filetree["path/to/file.ext"]
ERROR: Path components cannot contain '/' or '\' (got "path/to/file.ext")
Stacktrace:
 [1] error(s::String)
   @ Base ./error.jl:35
 [2] joinpath(path::DataSets.RelPath, xs::String)
   @ DataSets ~/data/.julia/packages/DataSets/DWg6S/src/paths.jl:28
 [3] getindex(tree::BlobTree{JuliaHubData.DataRepoCache}, name::String)
   @ DataSets ~/data/.julia/packages/DataSets/DWg6S/src/BlobTree.jl:326
 [4] top-level scope
   @ REPL[96]:1

You need to use the special @path_str macro or explicitly construct a RelPath for this to work. It'd be nice to just allow using the complete key as a string.

[mortenpi]

I am a little bit hesitant here, since it does look like it was a pretty conscious choice to not interpret slashes in Strings on the grounds that you may get weird platform-specific behavior. On unix-y systems, you might have a file with the name foo\bar, but we'd then interpret that as the path ("foo", "bar").

DataSets.jl/src/paths.jl

Lines 21 to 32 in c0e67b7

	RelPath(::AbstractString) = error("RelPath(::String) is not defined to avoid ambiguities between operating systems. Use the `path\"...\"` string macro for path literals.")
	RelPath(components::AbstractVector=String[]) = RelPath(convert(Vector{String}, components))
	# Path manipulation.
	function Base.joinpath(path::RelPath, xs::AbstractString...)
	for x in xs
	if '/' in x || '\\' in x
	error("Path components cannot contain '/' or '\\' (got $(repr(x)))")
	end
	end
	RelPath(vcat(path.components, xs...))
	end

On the other hand, such file names are effectively prohibited in DataSets.jl datasets because of this restriction, so it would probably not break anything if we just apply the same logic that path"" uses on strings passed to getindex.

Implementation-wise, this would be a pretty trivial change.