Feat/resume playback common play

[JarbasAl]

solves #18

depends on MycroftAI/mycroft-core#2660 and MycroftAI/mycroft-core#2674

includes/depends on #35 #32 - will need rebase before merging

This refactors the existing adapt intents to not capture too much:

• next/prev intents now need a context to ensure a playlist exists
• resume intent now needs a context to ensure playback is occurring

The skill will now handle common play queries, if the current status is paused:

• if the phrase is an empty string it means the intent was a generic match (play music/read audiobook...)
• if the phrae matches Resume.voc or Play.voc playback is resumed

This required some changes in core to avoid a call to stop audio service (which prevented resuming)