0.2.0

CHANGES:

• Use curl to download (#10, @emillon)
• Set switch base to prevent compiler downgrades (#12, fixes #7, @emillon)
• Add create --with=features (#14, @emillon)
• Set OPAMCLI=2.0 in opam subprocesses (#18, fixes #15, @emillon)
• Remove empty switch if create fails (#17, #18, #21, @emillon)