Add command line options struct for the validator

The limit for the number of struct members is parameterized using
command line options.

Add --max-struct-depth command line option.
Add --max-switch-branches command line option.
Add --max-function-args command line option.
Add --max-control-flow-nesting-depth option.
Add --max-access-chain-indexes option.
diff --git a/CHANGES b/CHANGES
index b957b38..5aa5699 100644
--- a/CHANGES
+++ b/CHANGES
@@ -7,6 +7,7 @@
    of environment variable SOURCE_DATE_EPOCH.
  - Validator implements relaxed rules for SPV_KHR_16bit_storage.
  - Fixes:
+   #500: Parameterize validator limit checks
    #508: Support compilation under CYGWIN
    #517: Fix validation when continue (or case) contstruct is also the head of a
      nested control construct.