Allow heading to end with \#

The problem was in a loop that skipped the optional closing hashes in a
heading like this:

    ### This is an H3 ###

Now it checks to see if a hash is escaped and if it is, treats it as a
rightmost character of the heading text, like this:

    ### This is an H3 #\##   ==>   ### This is an H3 ##

Fixes issue #146.

block.go

@@ -221,6 +221,9 @@ func (p *parser) prefixHeader(out *bytes.Buffer, data []byte) int {
 		}
 	}
 	for end > 0 && data[end-1] == '#' {
+		if isBackslashEscaped(data, end-1) {
+			break
+		}
 		end--
 	}
 	for end > 0 && data[end-1] == ' ' {
@@ -733,7 +736,7 @@ func (p *parser) table(out *bytes.Buffer, data []byte) int {
 	return i
 }
 
-// check if the specified position is preceeded by an odd number of backslashes
+// check if the specified position is preceded by an odd number of backslashes
 func isBackslashEscaped(data []byte, i int) bool {
 	backslashes := 0
 	for i-backslashes-1 >= 0 && data[i-backslashes-1] == '\\' {

block_test.go

@@ -132,6 +132,15 @@ func TestPrefixHeaderNoExtensions(t *testing.T) {
 		"*   List\n    * Nested list\n    # Nested header\n",
 		"<ul>\n<li><p>List</p>\n\n<ul>\n<li><p>Nested list</p>\n\n" +
 			"<h1>Nested header</h1></li>\n</ul></li>\n</ul>\n",
+
+		"#Header 1 \\#\n",
+		"<h1>Header 1 #</h1>\n",
+
+		"#Header 1 \\# foo\n",
+		"<h1>Header 1 # foo</h1>\n",
+
+		"#Header 1 #\\##\n",
+		"<h1>Header 1 ##</h1>\n",
 	}
 	doTestsBlock(t, tests, 0)
 }