Added option to supply 100x2 matrix for custom bias

[bfairkun]

Thanks for this nice tool. I was interested in changing the bias from which fragments are generated to better simulate how lower-quality RNA generates RNA-seq with a stronger 3' bias (https://bmcbioinformatics.biomedcentral.com/articles/10.1186/s12859-016-0922-z)... So the proposed change allows users to specify their own 100x2 dataframe (instead of reading in the cdna_pos_bias.csv or rnaf_pos_bias.csv) which they can generate however they please... For example, here is a plot of the cdna_pos_bias.csv and rnaf_pos_bias.csv data, as well as some other similar 100x2 dataframes I created from beta distribution (eg from the dbeta base R function).