DVPL-4290: Custom search commands add quotes around multi-valued field values

Fix: Added _to_string method which converts

+ bool ⇒ 't' | 'f'
+ str ⇒ str
+ number ⇒ str(number)
+ anything_else ⇒ repr(anything_else)

Signed-off-by: David Noble <dnoble@splunk.com>