Replace hard-coded messages with slot-based customization

[warpech]

Moved from Palindrom/palindrom-client#37

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@tomalec wrote:

Currently, we have hard-coded texts, and HTML structure for presenting messages at https://github.com/PuppetJs/puppet-polymer-client/blob/gh-pages/puppet-client.html#L72

Due to i18n, a11y, customization, theming, cheeper maintenance, etc. those should be customizable.

In my opinion the best way to do so is to use <slot>, we can provide fallback content inside, but tu CE consumer could simply write his/her own:

<puppet-client>
   <div slot="reconnecting-now" class="my-custom-styles">Łączę ponownie</div>
</puppet-client>

So we will no longer have to bother about how to show the message for the mobile, in different language, in different context, etc.

//cc @warpech @mnosek