placing a dash in a label stops further discovery

[dougbeal]

If you run mdrip test.md, you get no block found in given file https://gist.githubusercontent.com/dougbeal/f4d3671c94d010c35ea7c609f3c81fc5/raw/74326322309b0a64a5719b62d06baa4e1fb158d2/test.md

If you remove @dash-em, 2 block will be found.