No support for tmTheme background colors

[pouyakary]

• VSCode Version: All versions
• OS Version: macOS Sierra

.tmTheme definitions can have background colors specified with background keys such:

<!-- Number -->
<dict>
    <key>scope</key>
    <string>constant.numeric</string>
    <key>settings</key>
    <dict>
        <key>foreground</key>
        <string>#8fc2bbbb</string>
        <key>background</key>
        <string>#8fc2bb0f</string>
    </dict>
</dict>

and vscode provides on such support for this. A theme like NeonGlow that normally looks like this:

In vscode looks like:

[aeschli]

duplicate of #3429

[pouyakary]

@aeschli nice...