PHP version upgrade

Upgrade php version to ^7.1
Upgrade phpunit version to ^7
Fix unit tests

Added Filter interfaces

• Changed the way whitelisting and blacklisting work. Now you must use them like this:

new FilterPlugin([
            new WhiteListFilter(['some-site.com', 'me@email.com']),
            new BlacklistFilter(['blacklisted@some-site.com'])
])

• You can now create your own custom filters as long as they implement Openbuildings\Swiftmailer\Filters\ FilterInterface

Major Refactor

• Use PSR4
• Use PSR2
• Refactor for better code distribution
• 100 test coverage

0.1.2: Fix for empty recipients

Some transports do not check for empty recipient lists.
This way no email will be sent if there are no people to send it to,
without throwing any exceptions

0.1.1

Fix error when no cc or bcc are set

Initial Release

0.1.0

Update docs