Transition to use dashes for gflag flag separator

As per a recent dev list discussion we should transition to using and advertising dashes instead of underscores for multi-word gflag options.

A few things need to happen to make this a seamless transition, ideally landing in a single release cycle:

• Documentation in the /docs directory needs to be updated to use dashes.
• --help output needs to be changed to show dashes. This will require patching gflags. The upstream project would likely be receptive to such a patch, since they just landed support for dashes.