fix wordwrap of hyphens at limit

[JaredReisinger]

If the word-thus-far is exactly at the limit and wordwrap.Write() sees a hyphen, it writes the word to the buffer and then writes hyphen, exceeding the limit. This change fixes that behavior by instead adding the hyphen to the word as a normal character, performing the existing "is the word-in-progress too long" check to add a newline, and then writing the word-in-progress (without any length check).

It is still possible to see a hyphen beyond the limit, but that's a specific case of "don't break a word", where the hyphen is considered a part of the test to its left.

Fixes #66

[rwinkhart]

This fixes a bug I was experiencing when starting lines with hyphens.

I used the following simple program to demonstrate my issue:

func main() {
        fmt.Println(wrap.String(wordwrap.String("- This line wraps incorrectly on upstream reflow due to a silly bug regarding the handling of hyphens", 80), 80))
}

When run with upstream reflow, the line wraps incorrectly:

- This line wraps incorrectly on upstream reflow due to a silly bug regarding th
e
handling of hyphens

When run with this PR, the line wraps correctly:

- This line wraps incorrectly on upstream reflow due to a silly bug regarding
the handling of hyphens