Add twitterCard option to metadata

[abdonrd]

Option to update the theme-color meta tag.

<meta name="twitter-card" content="summary">

Reference: https://developer.twitter.com/en/docs/tweets/optimize-with-cards/overview/summary

Just check https://github.com/Polymer/pwa-helpers/commit/a4b56fd048cac57931e1aa7c76524687c502af9a. When https://github.com/Polymer/pwa-helpers/pull/47 is merged, I can rebase it to master.

[abdonrd]

About this: https://github.com/Polymer/pwa-helpers/pull/47#issuecomment-437193056

Why not have Twitter-specific when it is so common in applications?

[keanulee]

My thinking with this and themeColor (#52) is this:

• If the values for these tags don't change across different pages of the app (e.g. all pages use the same theme color, or all use Twitter summary card), then a static meta tag in index.html would be better than requiring JavaScript to be run (remember that opengraph tags are only useful if the crawler runs JS or the server pre-render's it).
• If the values do change, you still have the ability to modify these tags with your own JS code in updateLocation() callback. You might even want to set other Twitter-specific meta tags that we don't support.
• Perhaps it'd be useful if we exported the _setMeta function (renamed of course), then you can set whatever meta tags.

[abdonrd]

My use case is:

• I want to change the twitter:card depending of the page. I have summary as default, but I want to use summary_large_image in some pages. I do it at the same time I change the other metadata stuff.
• I want to change the theme-color when the user is offline.

And what updateLocation() callback?

[jsilvermist]

Perhaps it'd be useful if we exported the _setMeta function (renamed of course), then you can set whatever meta tags.

I think that's really the best option here.

[keanulee]

Added setMetaTag() to set arbitrary meta tag in #58