[spec] markdown support for descriptions/summaries/messages

[Ivshti]

We should support markdown (rather than a HTML subset) in various places of the app, including but not limited to:

• detail page: item description
• addon description (descriptionLong): https://github.com/Stremio/stremio-addon-sdk/issues/11
• user error messages: e.g. Stremio/stremio#402

related: Stremio/stremio#515 Stremio/stremio#402 https://github.com/Stremio/stremio-addon-sdk/issues/11

Those pieces of markdown should also support Stremio internal links (see Stremio/stremio#517), enabling complex use cases such as pointing to various filters in Discover, or even towards particular streams