Pgpc

The release includes more parser combinators:

- `ws`, `letter`, `digit`, `alphanum`
- `Parser.alt`, `opt`, `many` - parsers over parsers
- `Parser.map` to transform the result of a parser
- `Parser.replace_with`, `Parser.replace_lazy` to discard the value of the current parser
- `Parser.drain_next` to ignore the value of the next parser and use the current parser's value

Add tests and change the example from `README.md`

The version contains basic functionality to parse text data:

- `pgpc.scanner.Scanner` - a class that abstracts away a source that is being parsed;
- `pgpc.parser.Parser` - a generic object that represents a parser. A parser is just a function from `Scanner` to a generic value `V`;
- `pgpc.scanner.Position` - an object that represents positions of a parser;
- few simple parser combinators: `satisfy`, `any_char`, `char`.

The initial version contains basic functionality to parse text:

- `pgpc.scanner.Scanner` - a class that abstracts away a source that is being parsed;
- `pgpc.parser.Parser` - a generic object that represents a parser. A parser is just a function from `Scanner` to a generic value `V`;
- `pgpc.scanner.Position` - an object that represents positions of a parser;
- few simple parser combinators: `satisfy`, `any_char`, `char`.