TileLayer: add bounds prop

[DonNicoJs]

see #294 for the discussion

[bezany]

It is option of GridLayer (documentation).
It is can usage in vue2-leaflet with options prop. Example.
May be described details in documentation.

[DonNicoJs]

@bezany nice catch, I thought I saw a setter somewhere. I think the docs have a problem, related to docsify issue that does not show properly all the props coming from the mixins.

[stale]

This issue has been automatically marked as stale because it has not had recent activity. It will be closed if no further activity occurs. Thank you for your contributions.