Enable and disable directives are not symmetrical

[Zamiell]

For new checks and feature suggestions

• https://www.shellcheck.net/ (i.e. the latest commit) currently gives no useful warnings about this
• I searched through https://github.com/koalaman/shellcheck/issues and didn't find anything related

The Issue

Hello, and thanks for the excellent tool.

I am not sure whether this is a bug or a missing feature, but I expect enable directives and disable directives to have symmetrical syntax. But they don't, as illustrated by the following code snippet:

# shellcheck enable=check-extra-masked-return
# shellcheck disable=check-set-e-suppressed

The enable directive works, but the disable directive chokes:

In ./bash/.bash_profile_remote line 7:
# shellcheck disable=check-set-e-suppressed
^-- SC1073 (error): Couldn't parse this shellcheck directive. Fix to allow more checks.
                                           ^-- SC1072 (error): Expected '=' after directive key. Fix any mentioned problems and try again.

For more information:
  https://www.shellcheck.net/wiki/SC1072 -- Expected '=' after directive key....
  https://www.shellcheck.net/wiki/SC1073 -- Couldn't parse this shellcheck di...

The current workaround is to specify the short-hand code instead of the long name, like this:

# shellcheck enable=check-extra-masked-return
# shellcheck disable=SC2310

But doing this is confusing, because when you are enabling and disabling a bunch of rules, it looks buggy to have the long names for all of the enable directives and then using the short name for all of the disable directives.

I can try to submit a PR to fix this if needed.