Add multi-threading support

[IamLizu]

In case of batch checking with -f, we should add multi threading support.

• User can enable multi threading with -m, --multi-threading.
• By default maximum thread count should be limited and set to 5. Meaning wpdetect will create 5 thread at max by default.
• User can customize max thread count with passing number with thread flags like, -m 10.

PS: I feel this feature needs a bit more discussion.

[IamLizu]

I was having discussion with two of my colleagues ( @chandan9074, @joniyed ) and we have come to an agreement that we should not create new thread of each entry of the text file.

If we do, this may increase the overall operational cost. So, we shall create a thread every 3 or 5 entries for example.