v0.2 WIP

New Features / Changes:

• changed namespace from ImGui:: to ImPlot::
• renamed several API functions and structs for consistent style and clarity
• support multiple (up to 3) independent y-axes per plot
• multi-level digital plots
• negative data offsets
• optimizations for high density lines
• improved culling
• improvements toward ImGui C++11 conformance

Special thanks: @jpieper @ozlb @sergeyn @ocornut