use new universalmotif_df format

[snystrom]

Swap from my hacky solution to bjmt's nice one. https://github.com/bjmt/universalmotif/issues/13

To make this snappy, I'll make my functions private & still use them internally, but return the officially supported structure to users.

ToDo:

• [x] make as_universalmotif_df, as_universalmotif, update_motifs() private
• [x] return motif_df from runDreme, runTomTom
• [ ] change is_dreme_results and other family functions?
• [ ] use to_list when you'd otherwise call .$motif

Tests to write:

• runDreme & runTomTom return motif_df

[snystrom]

for runDreme output need to change:

• pvalue -> pval
• evalue -> eval
• qvalue -> qval (?)

[snystrom]

Issue: converstion to_list(extrainfo = TRUE) %>% to_df(extrainfo = TRUE) converts all extrainfo columns to character.