Automatically set isExtended()

[whikloj]

Currently you need to $bag->setExtended(true); before using bag-info tags and adding fetch files.

Instead it would be nice to allow you to do any of:

• $bag->setExtended(true);
• $bag->addBagInfoTag('External-Description', 'Some files');
• $bag->addFetchFile('http://example.org/some-file.txt', 'some-file.txt');

Any of these would enable the extended mode (if not enabled) and cause the tag manifests to be created.

[whikloj]

Partially resolved with https://github.com/whikloj/BagItTools/commit/b831fc9e98478ae2e0385a4470f5bc0900122808

[whikloj]

Fully resolved with https://github.com/whikloj/BagItTools/commit/633ce0e12f4c6a3df03dbf9004097f1bc8304840