v0.2.0 Getting Better

This is a complete re-write with breaking changes but more simple, with less options and more tests.

• Options: values, prefixes, keepLines, include, exclude, extensions, sourceMap
• Keywords: set, unset, if, elif, else, endif, ifset, ifnset, error

Now, varnames must match the regex /^_[0-9A-Z][_0-9A-Z]*$/, prefixed with '$' for replacement in the code.

Removal of comments no included, use rollup-plugin-cleanup instead.