Build the engine with custom modules in Visual Studio

First, compile the engine normally with:
```
scons custom_modules="path/to/your/modules" vsproj=yes
```
Then run the Visual Studio project. You can now rebuild the engine if
you need to make changes to custom modules directly within IDE.

methods.py

@@ -536,6 +536,7 @@ def build_commandline(commands):
                 '(if "$(PlatformTarget)"=="x64" (set "plat=x86_amd64"))',
                 'set "tools=yes"',
                 '(if "$(Configuration)"=="release" (set "tools=no"))',
+                'set "custom_modules=%s"' % env["custom_modules"],
                 'call "' + batch_file + '" !plat!',
             ]
 
@@ -555,15 +556,15 @@ def build_commandline(commands):
         # last double quote off, confusing MSBuild
         env["MSVSBUILDCOM"] = build_commandline(
             "scons --directory=\"$(ProjectDir.TrimEnd('\\'))\" platform=windows progress=no target=$(Configuration)"
-            " tools=!tools! -j" + str(num_jobs)
+            " tools=!tools! custom_modules=!custom_modules! -j" + str(num_jobs)
         )
         env["MSVSREBUILDCOM"] = build_commandline(
             "scons --directory=\"$(ProjectDir.TrimEnd('\\'))\" platform=windows progress=no target=$(Configuration)"
-            " tools=!tools! vsproj=yes -j" + str(num_jobs)
+            " tools=!tools! custom_modules=!custom_modules! vsproj=yes -j" + str(num_jobs)
         )
         env["MSVSCLEANCOM"] = build_commandline(
             "scons --directory=\"$(ProjectDir.TrimEnd('\\'))\" --clean platform=windows progress=no"
-            " target=$(Configuration) tools=!tools! -j" + str(num_jobs)
+            " target=$(Configuration) tools=!tools! custom_modules=!custom_modules! -j" + str(num_jobs)
         )
 
         # This version information (Win32, x64, Debug, Release, Release_Debug seems to be