Fix: add better attribute parsing helper function

[Nfrederiksen]

Issue: When parsing an attribute which may have a value that is comma-separated string, the current version cuts of parts of that value. eg. a line like #EXT-X-DATERANGE:ID="2",CLASS="com.apple.hls.interstitial",CUE="PRE,POST" would be parsed as

{
ID: "2",
CLASS: "com.apple.hls.interstitial",
CUE: "PRE" // expecting "PRE,POST"
}

This is due to the original parsing logic splits in "," first.

This PR fixes this by adding a better and more accurate parser function.