Environment Variable to not rewrite sources (or read config file)

[frisi]

would be great to have an option to tell mr.developer to not rewrite urls or ignore the config file in ~ or ~/.buildout

usecase:

• enabled rewrites for github using .buildout/mr.developer.cfg

[rewrites]
github =
  url ~ ^https://github.com/
  git@github.com:
  kind = git

• using mr.developer in the buildout of a package too

auto-checkout =  wm.sampledata
[sources]
wm.sampledata                       = git https://github.com/collective/wm.sampledata branch=python3

• building the package using tox (bobtemplates.plone generated package)
  -> asks me for my ssh key passphrase

eventually an environment variable could do the trick similar to this pr
eg MRDEVELOPER_DISABLEREWRITES is set, ignore rewrite settings of config file.