add ggplot and tidyverse other code improvements

[byandell]

Dan,
I have pulled your recent commits to intermediate, and have two extra commits of my own. You can the mediation_scan and ggplot_mediation_scan in particular, and some changes to other routines to make them more general. I will be adding my mediation_test and mediation_triad in coming weeks.
Brian

[byandell]

I made some more additions to add mediation_test and mediation_triad. I have improvements I want to make on mediation_triad, to use model fit, but it is a decent first attempt. Note that the Tmem68 example that was in library has the target (Tmem68) among the mediators, so one must be careful.

[byandell]

Dan,
Your change in mediation.scan (and elsewhere) from POS to MIDDLE_POINT broke the example for mediation.scan. It would be better to leave as POS for generality and change your code something like the following:
med <- intermediate::mediation.scan(
target = pheno_clin[,pname,drop=FALSE],
mediator = expr_med,
annotation = annot.mrna.chr11 %>% rename(pos = "middle_point"),
covar = covar,
qtl.geno = ap,
method = "double-lod-diff")