Allow per notification duration

[SamuelNitsche]

Allow per notification duration

This PR allows custom durations per notification.

Description

You can configure the duration like this:

toast()
  ->info('Bar', 'Foo')
  ->duration(1000)
  ->push();

Without the duration method, the default duration set in the config will be used.

You can even keep a notification forever using the following code:

toast()
  ->info('Bar', 'Foo')
  ->keep()
  ->push();

Related Issue

#23

Types of changes

• Docs change / refactoring / dependency upgrade
• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to change)

Checklist

• My code follows the code style of this project.
• My change requires a change to the documentation.
• I have updated the documentation accordingly.
• I have read the CONTRIBUTING document.
• I have added tests to cover my changes.
• All new and existing tests passed.

[usernotnull]

@SamuelNitsche thanks for your clean pull request, much appreciated 💪🏼

Everything looks in order, I'll just do a quick test and push a new release.