Support for hex, oct, and bin integers

[pelletier]

Add support for non-decimal integers. At the time of writing, this is an
unreleased backward-compatible feature of TOML:

  Non-negative integer values may also be expressed in hexadecimal, octal, or
  binary. In these formats, leading zeros are allowed (after the prefix). Hex
  values are case insensitive. Underscores are allowed between digits (but
  not between the prefix and the value).

  # hexadecimal with prefix `0x`
  hex1 = 0xDEADBEEF
  hex2 = 0xdeadbeef
  hex3 = 0xdead_beef

  # octal with prefix `0o`
  oct1 = 0o01234567
  oct2 = 0o755 # useful for Unix file permissions

  # binary with prefix `0b`
  bin1 = 0b11010110

Fixes #204