stricter requirements for directive comments

[nishanths]

For example, the current code in master considers comments of the form:

//exhaustive:ignore-more-text

to be a valid equivalent of:

//exhaustive:ignore

We don't wish to allow the above forms as an equivalent. This will be a breaking change for users who didn't use the strictly exact form.

That said, the following should continue to be allowed (notice instead of -):

//exhaustive:ignore more-text

See discussion here: #68 (comment).