Improve bold, italic and strikethrough parsing (GFM)

Now they are more similar to GitHub's algorithm.

Markdown.tmLanguage

@@ -419,7 +419,7 @@
 		<dict>
 			<key>begin</key>
 			<string>(?x)
-						\b(\*\*|__)(?=\S)								# Open
+						(?&lt;!\w)(\*\*|__)(?=\S)								# Open
 						(?=
 							(
 							    &lt;[^&gt;]*+&gt;							# HTML tags
@@ -456,7 +456,7 @@
 							  | (?!(?&lt;=\S)\1).						# Everything besides
 																	# style closer
 							)++
-							(?&lt;=\S)\1\b								# Close
+							(?&lt;=\S)\1								# Close
 						)
 					</string>
 			<key>captures</key>
@@ -546,7 +546,7 @@
 		<dict>
 			<key>begin</key>
 			<string>(?x)
-						(~~)(?=[^\s~])								# Open
+						(?&lt;!\w)(~~)(?=[^\s~])								# Open
 						(?=
 							(
 							    &lt;[^&gt;]*+&gt;							# HTML tags
@@ -926,7 +926,7 @@
 		<dict>
 			<key>begin</key>
 			<string>(?x)
-						\b(\*|_)(?=\S)								# Open
+						(?&lt;!\w)(\*|_)(?=\S)								# Open
 						(?=
 							(
 							    &lt;[^&gt;]*+&gt;							# HTML tags
@@ -964,7 +964,7 @@
 							  | (?!(?&lt;=\S)\1).						# Everything besides
 																	# style closer
 							)++
-							(?&lt;=\S)\1\b								# Close
+							(?&lt;=\S)\1								# Close
 						)
 					</string>
 			<key>captures</key>

tests/GFM.md

@@ -20,6 +20,11 @@ The word `complicated` must be neither bold nor italic below:
 perform_complicated_task
 perform__complicated__task
 
+But the first part below is italic and bold respectively:
+
+_perform_complicated_task
+__perform__complicated__task
+
 # Fenced Code Blocks
 
 ## In / Near List Items