Big rewrite: simpler API, more testable
Separated environment side effects from iterating function
Don't override existing environment variables!
Qualify enum names
Don't fail horribly on empty strings and newlines
Fix awful bug on already exhausted iterators
Added type aliases (readability counts!)
Assert that empty strings don't trip the line iterator
Modify README.md to reflect changes
Bump version number
Merge branch 'big-rewrite'

Conflicts:
	Cargo.toml
	src/lib.rs