Dequote CLOSED-CAPTIONS AttributeValue in STREAM-INF

Add subtitles, video, and audio group-id refs to StreamInfo

In the uri check method, I switched the library call "re" to the built-in "startswith".
I also used the same built-in method in the "remove_quotes" method. In the same file,
I changed the replaces of the method "string_to_lines" to "splitlines" where it has the
 same effect.

Improve readability in the parser.py file

Add the ability to set URI's when loading from a string

Main parser function now receives a function that can be
used to parse the line content and save it into the data store object.

Add basic support for custom tags

pytest-cov 2.6.0 has increased the version
requirement for the coverage package from >=3.7.1 to >=4.4,
which is in conflict with the version requirement
defined by the python-coveralls package for coverage==4.0.3.

Fix conflict of testing requirements

Document how to use a custom parser

datetime from standard library is able to perform the
same operations as arrow on m3u8 codebase.

Remove unnecessary dependency

Python 2.6 had it last release in 2013 and it is no
longer being maintained. Also it servers as a friendly
advice for those who still use Python 2 (https://pythonclock.org/)

Drop Python 2.6 support

Check if #EXT-X-PROGRAM-DATE-TIME is included in the
dumps output.

These attributes were missing in the output.

Missing EXT-X-MAP attributes on dumps

Catch ValueError and test for membership using not in

5.0.0 is out

Following the HLS spec, target duration is an integer
value.

Fix reused version