Various fixes

[pfitzseb]

This fixes #19 and makes showtable work for everything that's Tables.tableable (which includes Arrays, for example).

I'm not sure if this representation is what we want. Compare DataFrames default printing:

3×1 DataFrame
│ Row │ a       │
│     │ Union…⍰ │
├─────┼─────────┤
│ 1   │ 1       │
│ 2   │ missing │
│ 3   │         │

to

I was playing around with nothing => - or n/a and missing => ? but it's not clear to me if those are actually any better.