A GitHub-flavored Markdown autolink matcher: https://github.github.com/gfm/#autolinks-extension-.
The match_start function matches from the start of the string,
and returns None or the generated autolink, and the number of characters matched.
```rust use gfmautolinks::matchstart;
match_start("foo") // returns None
match_start("http://example.com more") // returns Some(("http://example.com", 18))
match_start("www.example.com more") // returns Some(("http://www.example.com", 15))
match_start("me@hotmail.com more") // returns Some(("mailto:me@hotmail.com", 14)) ```
The match_index function matches from a given index,
and also returns None or the generated autolink, and the number of characters matched.
If the index is not 0, it will also apply the rule,
that the autolink must be preceded by a whitespace character or one of * _ ~ (.
Invalid index will return None.
```rust use gfmautolinks::matchindex;
match_index("foo", 10) // returns None
match_index(" www.example.com", 1) // returns Some(("http://www.example.com", 18))
match_index("]www.example.com", 1) // returns None ```
Note, no HTML escaping is performed, e.g.
```rust use gfmautolinks::matchstart;
match_start("http://example.com?foo=bar&baz=qux") // returns Some(("http://example.com?foo=bar&baz=qux", 34)) ```
Originally adapted from comrak.