add support for title function for dynamic titles

[rkoval]

allows you to do something like this to show different titles for different conditions:

return new WebpackNotifierPlugin({
  title({msg}) {
    if (msg.startsWith('Error')) {
      return 'build error ❌';
    } else if (msg.startsWith('Warning')) {
      return 'build warning ⚠️';
    }
    return 'build complete ✅';
  },
});

[rkoval]

related to #48 i think

[Evilart86]

ty for this feature! its actually what I wanted)

[Gvozd]

@rkoval
Please rebase PR on current master, and update README.md
After I will merge PR

[Gvozd]

Rebased manually, and updated the documentation
v1.11.0