ScanCode Workbench should have a --disable-gpu option

GPU acceleration for Electron apps is problematic on certain systems, e.g. VM's, certain hardware configurations, etc. The following is the console output when attempting to run the develop branch of AboutCode-Manager on a Ubuntu 14.04 VM:
```
jono@ubuntu-14-04:~/newfolder/aboutcode-manager$ npm start

> aboutcode-manager@2.0.0-rc2 start /home/jono/newfolder/aboutcode-manager
> electron main.js

[16420:0524/165939:ERROR:buffer_manager.cc(438)] [.DisplayCompositor-0x30f6a91e6c00]GL ERROR :GL_INVALID_ENUM : glBufferData: <- error from previous GL command
[16420:0524/165940:ERROR:texture_manager.cc(2971)] [.RenderCompositor-0x39b3e1d6ac00]GL ERROR :GL_INVALID_ENUM : glTexImage2D: <- error from previous GL command
[16420:0524/165940:ERROR:gles2_cmd_decoder.cc(2291)] [.RenderWorker-0x39b3e1d6ba00]GL ERROR :GL_INVALID_ENUM : GLES2DecoderImpl::DoBindTexImage2DCHROMIUM: <- error from previous GL command
[16420:0524/165940:ERROR:gles2_cmd_decoder.cc(2295)] [.RenderWorker-0x39b3e1d6ba00]GL ERROR :GL_INVALID_OPERATION : ScopedTextureBinder::ctor: was unhandled
```
The set of error messages repeat ad infinitum.