Fix for #8777

Now filter name is trimmed and lowercased before evaluation for case
insensitive and whitespace trimemd check.

Signed-off-by: Oh Jinkyun <tintypemolly@gmail.com>
Since I worried about side effect. For white space I thought that it would be useless for any case, but some flags may be case-sensitive. `ParseFlag` might be used many places other than envfile, so I couldn't do lower-casing in it. But if you are sure such strict(case-sensitive) flag would not exist, lower-casing can be done at `ParseFlag`, of course. And the code will be much cleaner and easier to understand.