gosubst v0.2.1

A solid entry in the gosubst canon!

Updates for v0.2.1 include:

• changes to env var expansion such that ${ENV_VAR} is expanded, but $ENV_VAR is not – this allows us to use Go template variables;
• CLI options for only running env var expansion or only running templating, largely for debugging template development;
• the sh() template function now runs sh instead of /bin/sh;
• various fixes and cleaner errors, as well as updated documentation.