Add "entries" plugin option

[Vittly]

Plugin builds bundle for each set starting from single entry.

What if different bundles represent different pages with different redux code? Now we need to import all redux code because of single entry. We have no way to write conditional imports like "import A if building 1st bundle" and "import B if building 2nd bundle".

Simplest way to fix it is to declare different entries for different sets.

Which form is better:

// #1 - plugin option
new WebpackBemPlugin({
  entries: { // or entry, or setEntries, or sets, or i-dont-know
    // kayes are names of sets
    desktop: 'desktop.js',
    touch: 'touch.js'
  }
})

// #2 - webpack option somehow handled by plugin
{
  entry: {
    // kayes are names of sets
    desktop: 'desktop.js',
    touch: 'touch.js'
  }
}

// #3 - webpack option with "variable" like in output option
{
  // bad part is entry does not have variables in standard webpack
  //   we will need to do some hacking here
  entry: '[setName].js'
}

// #4 - define entry point at bemrc
// don't know how