[prefix-plugin] investigate Edge

[Pierre-Yves Gérardy]

I've forgotten to test the prefixer in Edge and it turns out it may be problematic.

• The prefixer detects a -webkit- prefix in Edge because it is the most prevalent one (using the current detection method). In other browsers, supporting the most prevalent prefix only is sufficient, but I don't know if it is also the case for Edge. -ms- prefixed properties may be proprietary/not on a standard track, in which case I wouldn't mind not supporting them... but I'll have to check it thoroughly to be sure.
• @media (min-resolution: 2dppx) detection doesn't work in Edge. It falls back to dpi, which works identically so there won't be user-facing issues, but it is still sub-optimal.

[Pierre-Yves Gérardy]

@media (resolution) is fixed, and both -ms- and -webkit- must be supported, see #33 for a follow up.