Be cleverer about includes

- `#Plomino import script` should not mess with
  `#Plomino import script_too`.
- Imports in scripts should be handled.
- Import a script the first time it's mentioned; thereafter ignore it.
- Allow `#Plomino include script`, to distinguish from Python's `import`
  statement.
Merge pull request #488 from plomino/jean-import-until-imported

Be cleverer about includes