NbCalendarComponent: allow month button background to be choosen

### Issue type

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

* [ ] bug report
* [x] feature request

### Issue description

**Current behavior:**

Color of month button in nb-calendar cannot be customized. I currently have to override _calendar-kit.theme.scss CSS directive with an important flag.

**Expected behavior:**

Have a theme variable for this button's color.

**Steps to reproduce:**

**Related code:**

See https://github.com/akveo/nebular/blob/master/src/framework/theme/components/calendar-kit/_calendar-kit.theme.scss

Here is my current CSS code (dirty fix):
```
nb-calendar-navigation button[nbButton] {
background-color: nb-theme(color-primary) !important;
}
```