feat(datepicker): pass current month value to 'markDisabled' callback

[maxokorokov]

Small datepicker enhancement: now able to access current month value both in custom template and markDisabled callback.

That allows for instance to disable completely days that are outside of current month and collapse them (see 'custom day view' demo):

[markDisabled]='markDisabled'

...

markDisabled = (date, current: {month: number}) {
  return date.month != current.month;
};

...

<template let-date='date' let-currentMonth='currentMonth'>
  <span [class.hide_day]='date.month !== currentMonth' ...>
</template>

...

.hide_day {
  display: none; // or visibility: hidden; to avoid line collapse
}

Contains an example for #847 and #753

[pkozlowski-opensource]

LGTM