Add Bikram Sambat as a calender option

[sodhancha]

Feature request:

Bikram Sambat is the prevelant calender currently used in Nepal. Even though not ideal and legacy we still have to use it for doing business in Nepal including generating Tax Invoices, Filing Taxes, Registering a company and any kind of compliance, tax or financial matters.

Logic for Bikram Sambat to A.D conversion is provided below in a jQuery plugin.
Hope someone from frappe team looks into Multi-Calendering and fixing Fiscal year ends according to that calender rather than A.D Calender.

Logic provided here:

https://github.com/leapfrogtechnology/nepali-date-picker

Bikram Sambat wiki page:

https://en.wikipedia.org/wiki/Vikram_Samvat