Implement GETENV facility for reading FROM and INTO clauses.

It might be important to be able to use the exact same pgloader commands
file but adapt its source and target depending on the environment where
the command is to be run (production, development, staging, etc).

Introduce the new sub-clause GETENV 'variable-name' to that effect.

The regression test facility that we have now isn't nearly sophisticated
enough to support this, so the feature isn't yet covered.