Single value subsetting (i.e. `[[`) interface

[nbenn]

All of base R data.frames, data.tables and tibbles support x[[i, j]] type subsetting. While I realise that there is quite a bit of heterogeneity (among the mentioned packages) in how edge cases are handled in subsetting and what type of arguments are supported, they all agree on support of "i, j" indexing. I therefore suggest adding this to fst as well in order to make the interface a bit more consistent.

I'm happy to submit a PR if this is of interest to @MarcusKlik.

[MarcusKlik]

Hi @nbenn, yes indeed, the x[[i, j]] generic is missing at the moment I see. For completeness, it would be nice to have it, a pull request would be much appreciated :-)