Specifying date range: add cal-like options -B and -A

[igneus]

From (an older version of) the cal man page:

-A number
             Months to add after. The specified number of months is added to the end of the display. This is in addition to any date range selected by the
             -y, -3, or -1 options. For example, “cal -y -B2 -A2” shows everything from November of the previous year to February of the following year.
             Negative numbers are allowed, in which case the specified number of months is subtracted. For example, “cal -y -B-6” shows July to December.
             And “cal -A11” simply shows the next 12 months.

-B number
             Months to add before. The specified number of months is added to the beginning of the display. See -A for examples.

Where to start:

• Start by adding to features/date_range_formats.feature a new scenario using one of the options and making it pass; then add another.
• Describe effects of each of the new options in spec/option_parser_spec.rb