BUGFIX: SNANA VARNAMES / VARLIST (read_snana_ascii)

The row in `SNANA`-formatted table files that indicates the names of columns can sometimes begin with the token `VARLIST` (not `VARNAMES`). `sncosmo` only checks for `VARNAMES`, causing it to die when it tries to read files that have the `VARLIST` token. This PR fixes this behavior by adding a `'VARLIST'` check on the same line as the `VARNAMES` check.