Install immunogenomics/presto to speed up FindAllMarkers

[kevinrue]

> # FindAllMarkers ----
> 
> seurat_markers_all <- FindAllMarkers(
+     seurat_object,
+     only.pos = TRUE,
+     min.pct = 0.25,
+     logfc.thres .... [TRUNCATED] 
Calculating cluster 0
For a (much!) faster implementation of the Wilcoxon Rank Sum Test,
(default method for FindMarkers) please install the presto package
--------------------------------------------
install.packages('devtools')
devtools::install_github('immunogenomics/presto')
--------------------------------------------
After installation of presto, Seurat will automatically use the more 
efficient implementation (no further action necessary).