Add feature for formatting and parsing english date

[aj3sh]

JavaScript lacks an easy way to format/parse Date objects. Let's create a utility from NepaliDate.

The APIs would look like the following:

• nepaliDateObj.formatEnglishDate("YYYY-MM-DD")
• nepaliDate = NepaliDate.parseEnglishDate("2018-12-48", "YYYY-MM-DD")

Use Cases

• Assists in initializing NepaliDate from an English date string.
• Facilitates obtaining the English date of a NepaliDate in a specific format.