Use RegExp for require.context to filter custom stylesheets (#2950)

ExtractTextWebpackPlugin extracts the content of loaded files, which means
it loads files loaded by require.context but not required after that.
However the former implementation is not aware of that.

require.context can have a RegExp to filter files to load. This change
gives a RegExp which matches with SCSSes with 'custom' prefix to
require.context to take advantage of the feature.