Add support for multi-character ValueSeparators

[kevin-montrose]

Implements multi-character value separators.

Visible changes:

• Options.ValueSeparator is now a string
  • It cannot be null or empty
  • It's first character cannot match any of the other special characters
  • If whitespace is being trimmed, it cannot contain whitespace

Internal notable changes:

• It's now possible for a the reader state machine to need to look ahead, which can increase the frequency of characters being left in the pending buffer
• Minimum memory needing for a read buffer is now 2 * (length of ValueSeparator) instead of 2