Choose header row

Some formats of CSV use a header with a few plain text lines at the top, before the line that defines the column names. I'd like to be able to choose which line is used as the "Header" in Table Tool. Perhaps by right-clicking a row.
Example attached.
[report-411951434879-2017-02-01.csv.txt](https://github.com/jakob/TableTool/files/1344677/report-411951434879-2017-02-01.csv.txt)

Start with the attached report-411951434879-2017-02-01.csv.txt file and inspect how Table Tool currently identifies the header row and exposes row actions. Define how a user selects a different row, then verify that the chosen row becomes the column header while the preceding plain-text lines remain excluded from the table data.

Written by the indexing model from the issue text.