support comments

[mwotton]

I have some large chunks of pre-existing sql that are heavily commented, to make it clear what's going on. Unfortunately hasql-th doesn't seem to cope:

select   "bar" :: text as subnets

works fine, but

select   "bar" :: text as subnets -- foo

gives me this:

 |         
1 | select   "bar" :: text as subnets -- foo
  |                                   ^
unexpected '-'
expecting end of input or white space

[nikita-volkov]

This will require an extension to the parser. I'm leaving this for PRs.