Add support to minView in nb-datepicker

### Issue type

**I'm submitting a ...** (check one with "x")

* [ ] bug report
* [x] feature request

### Issue description

**Current behavior:**
Currently we can setup **startView** but we cannot set minView / minViewMode same way as bootstrap picker for selecting only year or month and year.

**Expected behavior:**
We can set **minView** to "month" or "year" and selection will end at defined point.

**Steps to reproduce:**
Build in behaviour.

**Related code:**

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### Other information:

**npm, node, OS, Browser**
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**Angular, Nebular**
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