Expand macro functionality

MODSEC-64: Some ideas for making macros more useful:

• %{&COLLECTION} -- count of COLLECTION variables
• %{t(lowercase,ARGS)} -- Allow transformations in macros
• %{rand(N)} -- Allow other functions like random values (should be extendible via API)
• Allow defining macros -- SecMacro FOO "foo"
  These are config time macros vs runtime. Maybe use %{{CONFIG_MACRO}}?
• Allow defining macros with parameters -- SecMacro FOO(bar) "foo%{bar}"
• Allow macro expansion in TARGETS (at least config-time) -- ARGS|%{{CUSTOM_FIELDS}}
  {noformat}
  SecMacro CUSTOM_TARGETS "ARGS|!ARGS:description"
  ...
  SecRule %{{CUSTOM_TARGETS}} "foo" "..."
  {noformat}