fix(token): throw on reserved token names instead of silently corrupting morgan

[Samiullah-2004]

Fixes #265

Calling morgan.token('token', fn) (or 'format'/'compile') silently
overwrote morgan's own internal functions, since token() just does
morgan[name] = fn. This meant registering a token named "token" would
destroy morgan.token itself, causing every subsequent .token() call
to fail with an unrelated TypeError, with no indication of the real cause.

This PR:

• Adds a check in token() that throws a clear TypeError when a
  reserved name (token, format, compile) is used
• Adds regression tests, including one that verifies morgan.token
  remains a valid function after a rejected registration (the exact
  failure mode from the original bug report)
• Documents the reserved names in the Readme, as requested by
  @dougwilson in the original issue thread

All 91 existing tests still pass.