Restore ability to set config from ENV vars.

On reflection, I have repented. However, the change to how ENV vars
were read previously is that they are expected to be lowercase, in
contrast to popular implementation. But read

    http://pubs.opengroup.org/onlinepubs/007908799/xbd/envvar.html

So, for the executable, each config setting is now set in the following
order of priority, with a mixture of methods possible:

- command-line parameters (top priority; overrides others)
- ENV vars
- defaults

Note that when using Redimap as a library, only the configure block is
used; there is no reading of ENV vars, and of course no reading of
command-line parameters.
Document restored ability to set config using ENV vars.

Also provide full list of available ENV vars.
Merge branch 'configuration' into develop
Bump version to v0.5.0.
Merge branch 'release/v0.5.0'