Add the number of the problem in the book

The book is "Cracking the Coding Interview, 5th Edition"
An algorithm to return a bit field.

Implement a function which returns a value corresponding bit positions for a given value
which starts at 'start' with width 'width'

For example,
val = 0xABCD8364
get_bit_field(val, 4, 4) == 0x6
get_bit_field(val, 8, 12) == 0xD83