[r3wretrhy]

Fixes #440

Turned off interpolation in the config parser, as suggested in the issue. With that a percent-encoded mirror login (the form the README recommends for --mirror) can sit in setup.cfg or ~/.nodeenvrc without breaking nodeenv in that directory.

One side effect worth knowing: if someone wrote %% to work around the crash, they'll get %% back. I put that in the changelog entry.

There's a small test next to the quote-stripping one. Unit tests and flake8 pass locally; the Windows, zsh and fish shell tests were skipped since I don't have those here.

• Tests cover the change
• tox passes: the tests and flake8
• CHANGES has an entry under Version [unreleased], if nodeenv's behavior changes
• README.rst is updated, if the change affects what it documents
• The title has the form type(scope): subject

[r3wretrhy]

Rebased onto current master; kept the % config changelog entry alongside the recent -p PATH fix.