Feature/plugin v2

[hkalexling]

A new version of the plugin system that supports the following additional features:

• Manga search with search terms
• Subscriptions and auto update
• A new helper function mango.settings(key). See https://github.com/hkalexling/mango-plugins/wiki/Development-Guideline-v2-(RFC) for details.

To target the new v2 API, plugins must put 2 in the api_version field in the info.json file. For backward compatability, any plugin without the api_version specified will be treated as a v1 plugin.

Closes #24. Related getmango/mango-plugins#10 and getmango/mango-plugins#7.

[Leeingnyo]

It's a very impressive and fantastic feature! It seems you have done really hard work, Thank you.
I really liked the component pattern that applied to subscription-manager and plugin-download, it seems toward to a standalone client.
I have read most of codes (except plugin-download, api :p I just had a glance at them), and they looks good to me.

Here's some notices.

1. After applying filters at plugin-download.html, sorted columns are not preserved though the UI is set to sorted.

First, get search result and sort them (I searched New Game! for example). the table shows well sorted columns started with 1.

After apply filter 'volume = 6' (min 6, max 6), the UI shows that the results are sorted by chapter though they are not actually.

This occurred too when I select another manga at the search result.

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I would propose these, too.

1. Show a confirm modal when deleting subscriptions
2. In subscription detail modal, show a manga title at somewhere.

Thanks again for your hard work.

[hkalexling]

@Leeingnyo Thanks for reviewing the PR! These are all good points that can help improve the user experience. I will update the PR.