Bump github.com/russross/blackfriday/v2 to v2.1.0

See release notes at
https://github.com/russross/blackfriday/releases/tag/v2.1.0

This also drops two indirect dependencies.

Signed-off-by: Tobias Klauser <tklauser@distanz.ch>

go.mod

@@ -2,8 +2,4 @@ module github.com/cpuguy83/go-md2man/v2
 
 go 1.11
 
-require (
-	github.com/pmezard/go-difflib v1.0.0 // indirect
-	github.com/russross/blackfriday/v2 v2.0.1
-	github.com/shurcooL/sanitized_anchor_name v1.0.0 // indirect
-)
+require github.com/russross/blackfriday/v2 v2.1.0

go.sum

@@ -1,6 +1,2 @@
-github.com/pmezard/go-difflib v1.0.0 h1:4DBwDE0NGyQoBHbLQYPwSUPoCMWR5BEzIk/f1lZbAQM=
-github.com/pmezard/go-difflib v1.0.0/go.mod h1:iKH77koFhYxTK1pcRnkKkqfTogsbg7gZNVY4sRDYZ/4=
-github.com/russross/blackfriday/v2 v2.0.1 h1:lPqVAte+HuHNfhJ/0LC98ESWRz8afy9tM/0RK8m9o+Q=
-github.com/russross/blackfriday/v2 v2.0.1/go.mod h1:+Rmxgy9KzJVeS9/2gXHxylqXiyQDYRxCVz55jmeOWTM=
-github.com/shurcooL/sanitized_anchor_name v1.0.0 h1:PdmoCO6wvbs+7yrJyMORt4/BmY5IYyJwS/kOiWx8mHo=
-github.com/shurcooL/sanitized_anchor_name v1.0.0/go.mod h1:1NzhyTcUVG4SuEtjjoZeVRXNmyL/1OwPU0+IJeTBvfc=
+github.com/russross/blackfriday/v2 v2.1.0 h1:JIOH55/0cWyOuilr9/qlrm0BSXldqnqwMsf35Ld67mk=
+github.com/russross/blackfriday/v2 v2.1.0/go.mod h1:+Rmxgy9KzJVeS9/2gXHxylqXiyQDYRxCVz55jmeOWTM=