Upgrade blackfriday to v2 and fix underscore italics (#350)

Now that we have a go.mod, we can properly upgrade to blackfriday/v2

The output remains the same, but this PR brings back italic links with '_',
placing them into the link since this seems to be the more proper way.

Fixes #182

examples/mutexes/mutexes.go

@@ -1,6 +1,6 @@
 // In the previous example we saw how to manage simple
 // counter state using [atomic operations](atomic-counters).
-// For more complex state we can use a <em>[mutex](http://en.wikipedia.org/wiki/Mutual_exclusion)</em>
+// For more complex state we can use a [_mutex_](http://en.wikipedia.org/wiki/Mutual_exclusion)
 // to safely access data across multiple goroutines.
 
 package main

examples/mutexes/mutexes.hash

@@ -1,2 +1,2 @@
-7cb6349117087c78ddb71c240e988ec8281c8952
-0WEmOOjoCjp
+253b089b8145fc57a90ae4024346b6db2ec1659b
+CHCDredHCOz

examples/rate-limiting/rate-limiting.go

@@ -1,4 +1,4 @@
-// <em>[Rate limiting](http://en.wikipedia.org/wiki/Rate_limiting)</em>
+// [_Rate limiting_](http://en.wikipedia.org/wiki/Rate_limiting)
 // is an important mechanism for controlling resource
 // utilization and maintaining quality of service. Go
 // elegantly supports rate limiting with goroutines,

examples/rate-limiting/rate-limiting.hash

@@ -1,2 +1,2 @@
-c2d4dd4c2121e61395db186e3f15ce8cb3acf643
-20c_m1AtOEI
+c7063265708287744ea172ed9ed1390043140718
+GXjXHfnKFXg