Support arbitrary separators in `read_csv`

[apcamargo]

Description

TSV files are pretty common, but I couldn't make them work with polars-cli. Apparently, the read_csv function of polars-cli doesn't take the same arguments as the one in Python (maybe it doesn't support arguments at all?)

From what I've seen in table_functions.rs, read_csv uses a LazyCsvReader, which apparently supports the separator argument. I can't say I'm very familiar with Polar's code, though. I might be missing something obvious.