fix bug about implicit editable mode installation with VCS URL.

there's three problems.

1.
extra_args is modified after appending to cmd, so current behavior of the pip module(normal install, not editable mode) on VCS URL is different from its docs if user doesn't add '-e' extra_args explicitly.

2.
pip requires '-e' argument for 'git+git@...' form URL. because of 1st problem, pip will fail if user doesn't add '-e' extra_args explicitly.

3.
and...I think it's necessary for some developers(for example, me.:-)) to install package with normal vehavior (not editable mode) from VCS URL, so '-e' extra_args should be added explicitly(except 'git+git@... form ' URL).