feat(modal): Support custom window html

[nl5887]

the window attribute allows to use separate custom windows per modal
instance

[bekos]

@nl5887 Thx for the PR, but creating another directive just to override the template is not the best solution, as this is going to be hard to maintain, ie keeping in sync your custom directive with the official one. Please check #2057 if this is what you are after.

[nl5887]

Hmm, reason I've implemented it this way is to have the possibility (full control) to have an own controller, eg overwriting or customizing behaviour like the close.

[bekos]

I see, but the way to go, is to override/communicate with what modal decides to expose, and not trying to replace the whole thing. If this is what you want, you can create your own directive anyway, without depending on $modal's behaviour.

[pkozlowski-opensource]

@nl5887 thnx a bunch for this pull request, but finally I've decided to merge #2057 instead. Hope to see other contributions from you, though!

[nl5887]

Ok, thnx.