add mode support for summarise

[jacopomazzoni]

this solution:
https://www.reddit.com/r/RStudio/comments/15c4wuh/mode_of_categorical_variable_in_dplyr/

has been around for a while and seems to work excellently.

is there any reason why it shouldn't be included by default into the library?

I think mode is a big absence from the default options.

sorry I know this isn't the proper procedure but it's such an easy add for you and so much work for me.

thanks