Feature request: obey env var for verts path

[kergoth]

virtualenv obeys WORKON_HOME, I'd suggest something similar for vert, e.g. VERTS_HOME. I'd also suggest changing the default from ~/.verts to ${XDG_DATA_HOME:-~/.local/share}/verts, but that's less of an issue if I can use the env var to move it there :). See XDG Base Directory Specification. Thanks much, I'm finding vert quite useful.