Make Wrapf a bit more useful by accepting a Position.

Call UnmarshalText once per token.

Create a dedicated package in the _examples directory.

Improve error reporting.

Previously, errors for branch lookaheads were discarded and
a generic "unexpected X" was returned. Those errors are now
tracked and reported if they are the "deepest" error.

Ensure lexer state is propagated out of ParseFromLexer().

Merge branch 'sleepinggenius2-empty-disjunction-node'

* sleepinggenius2-empty-disjunction-node:
  Adds test for issue alecthomas#60
  Adds error for empty alternative expression

Minor tweaks.

Add support for a full set of match modifiers (*, +, ? and !).

- `*` Expression can match zero or more times.
- `+` Expression must match one or more times.
- `?` Expression can match zero or once.
- `!` Require a non-empty match (this is useful with a sequence of optional matches eg. `("a"? "b"? "c"?)!`).

Test that lookahead disambiguates by type correctly.