Enables user to configure perl executable used during build and test

This patch enables users to configure the which perl executable Dist::Zilla
will use to perform build, test and install actions. It will use the value in
the DZIL_PERL env variable or default to $^X if DZIL_PERL isn't set.

This allows developers to use dzil installed under one version of
perl/local::lib, on projects which require a different version of
perl/local::lib to build/test/install. This has a couple of advantages:

    - you only need to install Dist::Zilla (and plugins and dependencies) once
    - you can keep your active project's installed modules to a minimum, so
      testing dependencies with "rm -rf .local-lib && dzil listdeps | cpanm &&
      dzil test" becomes managable
    - and testing with multiple versions of perl doesn't require a full
      Dist::Zilla toolchain install

If you are receptive to this new config option, I would be happy to update docs
and send pull requests for some of the more popular plugins where $^X is used.
I'm also open to alternative solutions.

I considered implementing as a dzil command line argument, but the env variable
is consistent with local::lib's method of modifying the environment. I don't
believe this should be implemented as a plugin, because it is really developer
specific, not project specific. Most projects expect developers to share a
dist.ini, and if I add

  [PerlExec]
  perl=/home/mgrimes/perl5/perlbrew/perls/perl-5.14-clean/bin/perl

to it is highly unlikely to work for another contributor.