Prevent error caused by passing named arguments to read.cross.tidy

For example:

```r
x <- read.cross("tidy", "", genfile=c(geno="hyper_tidy_gen.csv"),
                mapfile="hyper_tidy_map.csv", phefile="hyper_tidy_phe.csv",
                genotypes=c("BB", "BA", "AA"))
```

```
## Error in read.table(sep = ",", na.strings = c("-", "NA"), row.names = 1,  : 
## unused argument (file.geno = "hyper_tidy_gen.csv") 
```

Granted this isn't something many people would encounter. I ran into it while trying to create multiple crosses from a list of input files.