Lexer global state management

[Morikko]

Solve: #108

1. luqum.thread.parse uses luqum.parser.lexer
2. Introduce luqum.parser.parse that use the correct lexer automatically
3. Add documentation

I tried to make the simplest implementation that avoids any breaking change.

[alexgarel]

Great PR ! Thanks @Morikko

[alexgarel]

@mmoriniere this is ready !