Feature request: support case-insensitive regex

 * Extension version: v7.0.109
 * Browser name & version: Vivaldi 2.3.1440.57
 * Operating system & version: Windows 10 x64 1809

Currently the whitelist doesn't support regex like this:
```regex
/pdf/i
```
Therefore, it has to be written like this:
```regex
/[pP][dD][fF]/
```
which looks ugly.