Support for loading files wrapped in .gz and .bz2 (also file metadata)

[nickzoic]

Also raised in #8 : it's pretty normal for people to keep big .txt, .csv or .fq files hanging around as .csv.bz2 or .csv.gz etc. It'd be good to have support for this and even better to have support for this baked in to a file reader superclass so that every individual file reader plugin doesn't have to implement this itself.

[nickzoic]

While I'm talking about a file loader superclass, it seems like file metadata is going to be pretty common for when loading many files. You could do this by having a "filename" column and then joining on filename with a metadata DataTable but that's kind of clunky and it'd be nicer to have a common mechanism across file loaders.

[nickzoic]

this may or may not prove to be useful, one problem is that lots of file loading libraries have their own handling for bzip2 etc so it's hard to know where to slice this.