Add default gene names for probeToMeanPromoterMethylation

[katehoffshutta]

Currently a user has to put in a list of unique genes. We need a default option that parses said list from the input probemap so that this isn't required.

[katehoffshutta]

In addition to this option, we also need to add documentation describing the format for the list of input genes.

[katehoffshutta]

Resolved in #65