Support reading random rows in read_csv

[seven7e]

It is very common to read random rows in a large csv file, typically for testing with a small dataset, or fit the limit of memory. The parameter nrows is used for read the first n lines, but I didn't find any feature to read random lines. Such parameter might be named keeprows (opposite to skiprows), which supports:

• int, e.g. keeprows=100 means keep 100 random lines (uniformly)
• float in (0, 1), e.g. keeprows=0.05 means keep 5% of total lines
• list of int(or iterable), e.g. keeprows=[1, 3, 8] mean to keep line 1, 3, and 8

[jreback]

duplicate of #3132

yes, this is a good idea.