Fix the standalone REPL to pr-str values instead of print them. (`print`
led to a lack of quotes around strings.)  Value-printing in the standalone
REPL now matches nREPL mode, where the nREPL remote pr-str's values for us.

Add a new option :value-to-string and a corresponding command line option
--value-to-string.  These can be used in standalone mode to replace pr-str.

Clarify --print-value help string.
Fix a missing newline when using --color with the standalone REPL.
The original output was:

  user=> (/ 1 0)
  #<ArithmeticException java.lang.ArithmeticException: Divide by zero>user=>

print-err doesn't print a newline, so use (or print-err print) and print
a newline after it.
Fix --help: color is available in standalone mode.

(The only difference compared to nREPL mode is that stderr output
in the standalone REPL is not yet highlighted red.)