Support global installations of luaenv

[devurandom]

It appears that luaenv does not support being installed anywhere else but in ~/.luaenv:

docker run -ti debian:8 bash -c 'apt -y update \
 && apt -y install git \
 && git clone https://github.com/cehoffman/luaenv.git /opt/luaenv \
 && git clone git://github.com/cehoffman/lua-build.git /opt/luaenv/plugins/lua-build \
 && export PATH=/opt/luaenv/bin:$PATH \
 && eval "$(luaenv init -)" \
 && luaenv install 5.3.3'
> ...
> luaenv: no such command `install'

It would be great if it could search for plugins also in its own installation path, i.e. /opt/luaenv in this case.