Method to get column names in a FITS table

[kbarbary]

I was surprised that this doesn't already exist, but it's straightforward to add.

Column names are already determined as part of show(::IO, ::TableHDU) and show(::IO, ::ASCIITableHDU). One would just have to pull out some of that code into a separate function.

colnames might be a good function name for this. DataFrames extends Base.names for this purpose, but the definition of Base.names is, I think, inconsistent with this usage.