change has_header option to header

[kshefchek]

Currently to specify whether a tsv has a header or not we have a has_header option. During the kozathon we determined just "header: true|false" is the more expected option for this.

[kshefchek]

See also how header, names, and skip_lines are handled in pandas - https://pandas.pydata.org/docs/reference/api/pandas.read_csv.html we may want to match this behavior for consistency.

[caufieldjh]

I like the pandas approach because then you don't need the skip_lines option. It would probably save a lot of parsing issues if Koza behaved much like pandas, at least in terms of input parsing options.