Document --version
jv_invalid() shouldn't allocate

jv_invalid() should behave like jv_invalid_with_msg(jv_null()), and
neither should allocate memory, because neither ought to need to.
exec_stack.h was not using jv_mem_realloc()
Add jq_handle concept
Add --args option
Add jq_runtime_flags enum type
Add _fopen and _popen builtins
Better handling of popen in f_read
Document --args option
Document --allow-* options
Add `write`
--allow-write implies --allow-open
Document `read`, `write`, `fopen`, and `popen`