YCMEPHelper: Add developer mode

Each subproject now has a YCM_EP_DEVEL_MODE_<Proj> cached variable that
is used to enable devel mode targets and disable the update target.

The YCM_EP_EXPERT_MODE enables the update targets for the projects in
development mode.

The YCM_EP_MAINTAINER_MODE enables all the targets for all the projects.

Fixes #31