Move calendar generation logic into nepcal package

[Srishan]

Currently, the CLI does the manual work of creating the calendar, but it can be done by the nepcal package for any date. An interface like the following could be exposed:

type Calendar struct {
  // Use a sentinel to denote empty days
  weeks [][7]int

  // could contain some information derived from the time. 
  // expose via methods
 StartWeekday Weekday
 NumDaysInMonth int
}

func (t Time) Calendar() Calendar {}

[Srishan]

Closed by #70