Add new metadata field in anticipation of chocolatey.org

[alexchandel]

#18 mentions three useful fields,

• [ ] replaces
• [ ] provides
• [ ] conflicts

which were not added at the time, because chocolatey.org presently ignores them. However, many packages have infrequent release cycles, and preemptive adoption will ease the transition to virtual packages by guaranteeing the existence of some infrastructure, so I propose these fields, particularly provides, be added now.

This is applicable to packages like cmake.portable, handbrake.portable, and others, as well as the corresponding .install packages.