Expose Electron's clipboard API to extensions

[MattIrv]

When writing extensions right now we have to use a node module such as copy-paste in order to access the clipboard. This leads to issues such as microsoft/vscode-mssql#1059 and microsoft/vscode-mssql#1036 since it is not closely integrated with VS Code.

For extension authors it would be helpful if VS Code exposed some or all of Electron's clipboard API since it would make it much easier to get copy/paste working in a way that integrates nicely with VS Code and Electron.

[jrieken]

see #217

[jpoon]

#217 has been closed as a won't fix but any chance we can revisit it? @octref's comments are still relevant. vscodevim users are also asking for it VSCodeVim/Vim#3038 and we'd hate to have to re-write the wheel as @rebornix has already done a great job in implement RTF clipboard in vscode itself.

[octref]

#217 is reopened :)