Ignore Visual Studio Code files (ros-navigation#299)

Using Visual Studio Code, I've been opening just the directories that I'm interested in, avoiding the output directories (install, build, and log). In this configuration, when debugging various modules, the VS Code app will put .vscode files in each source directory being debugged. This causes the 
 directories to be highlighted in the editor indicating that files have been added. With this addition to .gitignore, these files will be ignored and the VS Code editor will more accurately reflect any changes to the workspace.

.gitignore

@@ -28,8 +28,10 @@
 *.out
 *.app
 
-#colcon output
+# Colcon output
 build
 log
 install
 
+# Visual Studio Code files
+.vscode