Possibility to add custom template in CalendarModal header

[fernandocode]

Ionic version: (check one with "x")
[ ] 2.x
[x ] 3.x
[ ] 4.x

Ion2-calendar mode: (check one with "x")
[ ] components mode
[x ] modal mode

I'm submitting a ... (check one with "x")
[ ] bug report
[ x] feature request
[ ] help me

What I would like it to be possible to add a sub header (below the header and above the weeks), permintindo add custom controls to the modal, in my case I would like to add buttons with the selected dates, and allow to click them to go the corresponding date , is a specific case for my requirements. So that's why I suggest creating a customizable area instead of functionality.

I think this could be done through <ng-content>.

I can send a pull request with this functionality.