Add checkbox for copying by keyboard shortcuts & "Ignored sites" list

[Yang-Pi]

Experience showed that copying text as a plain one doesn't use every time, so it will be better to turn on it only when it will be needed. There are two ideas:

1. Create an option in a right-click menu. User case is select text -> right-click -> choose "Copy as a plain text" option.
   Prons: simple implementation;
   Cons: there are a lot of apps like this already.

2. Create an option in a right-click menu
   + add a checkbox to turn on copying as a plain text by classical shortcuts
   + add "Ignored sites" list
   Prons: complex logic for convenient work
   Cons: it's not an obvious solution

The issue name shows my choice.