same mention name multiple times not working properly

[vjey23]

@omm test string @omm results in @omm@omm test string @omm after expand

"mention": func(b string) string {
				// Take substring from index 4 to length of string.
				return fmt.Sprintf(`<a href="/test/%[1]s">%[1]s</a>`, b)
			},

[huumn]

I fixed this bug here https://github.com/huumn/anno ... it required a modified bytes.Fields to keep track of indices ... I couldn't think of a better way to fix the issue given the time I had.

[matryer]

@huumn Can you send a PR in so we can all have that fix please?

[k1ng440]

@matryer please check the PR