Templates: Add PropEr support

[oubiwann]

E.g., in an existing project:

rebar3 new lfe-proper-lib [NAME]
rebar3 new lfe-proper [NAME]

There's no equivalent of the first one in rebar3; it would just be the same as an lfe-lib project, except that the deps would be set up in rebar.config ... and maybe a default PropEr test added.

The second one would be an LFE version of rebar3 new proper (creating a single PropEr test file).