Add support for default values via environment variables

[asiminiceanu]

Add support for default values at start / build time for uiSchema, schemaUrl or some other labels in the page that are not part of the uiSchema

This is useful for scenarios where you just need to publish some specs and allow people to view them and try them out.

[codecov[bot]]

Codecov Report

Patch and project coverage have no change.

Comparison is base (c76dc9a) 0.00% compared to head (899a1df) 0.00%.

:exclamation: Current head 899a1df differs from pull request most recent head 96131d1. Consider uploading reports for the commit 96131d1 to get more accurate results

Additional details and impacted files

@@      Coverage Diff      @@
##   master   #604   +/-   ##
=============================
=============================

Help us with your feedback. Take ten seconds to tell us how you rate us. Have a feature suggestion? Share it here.

:umbrella: View full report in Codecov by Sentry.
:loudspeaker: Do you have feedback about the report comment? Let us know in this issue.

[asiminiceanu]

@shanejonas can you have a look at this?

[shanejonas]

sorry this took so long to take a look at. tested it out by hand looks like its all working properly. thanks for the contribution.