SC3037: echo flags

For bugs

• Rule Id (if any, e.g. SC1000): SC3037
• My shellcheck version (shellcheck --version or "online"): 0.7.2
• The rule's wiki page does not already cover this (e.g. https://shellcheck.net/wiki/SC2086)
• I tried on https://www.shellcheck.net/ and verified that this is still a problem on the latest commit

Here's a snippet or screenshot that shows the problem:

#!/bin/sh
/bin/echo -e text

Here's what shellcheck currently says:

/bin/echo -e text
          ^-- SC3037: In POSIX sh, echo flags are undefined.

Here's what I wanted or expected to see:

/bin/echo, /usr/bin/echo, ... are binaries and are not builtin shell commands and are separate from the shell. shellcheck shouldn't warn about undefined flags for these binaries.