Support multiple characters for `comment` in read_csv

my code is:
```python

vcf = pd.read_table("com8-2.N85-2.merge.snp.vcf", header=0, sep="\t", comment="##")

```
and get this error:

`ValueError: Only length-1 comment characters supported`
the error means the length of comment must be one ? why not allowed for  two characters ?

The python version is python3.6, 
pandas version is pandas-0.20.3

thanks~
Si