Expand the input buffer in `dt_read_csv`

[Kiyoshika]

Right now the input buffer when reading a line from file is 4096 bytes (including null term), but this is limiting for larger files. Will need to maintain one heap-allocated buffer and keep reusing per line (resizing if necessary) to support larger input files.

[Kiyoshika]

NOTE: As of fc20acc this now also applies to dt_table_to_csv