Choose a prefix for the password recovery

[kimci86]

This is a quick hack to be able to set a prefix for the password recovery.
This will be replaced by a proper mask feature.

Example:
./bkcrack -k f27e56a8 91e5b1ea 709073e0 -r 12 ?p -m "I remember the beginning but then"

[23:53:36] Recovering password
length 0-6...
length 7...
length 8...
length 9...
[23:53:36] Password
as bytes: 49 20 72 65 6d 65 6d 62 65 72 20 74 68 65 20 62 65 67 69 6e 6e 69 6e 67 20 62 75 74 20 74 68 65 6e 20 49 20 66 6f 72 67 6f 74 
as text: I remember the beginning but then I forgot

@linux-mining This should work for you.
Related to #55

[kimci86]

@linux-mining
You can download compiled binaries from GitHub Actions to test this branch, or compile from source yourself if you prefer.

To make the recovery fast, choose the minimal charset.
For example, if you know the characters left to be recovered are in set abcd1234, use option -r 12 abcd1234.
You might want to increase the value 12 in this example if there are more than 12 characters left to be recovered after the known prefix.

[linux-mining]

Once again thank you for the binary files, this makes it easy as my source control and compiling is a bit rusty. I'm on Ubuntu.
I will report back.

Thanks

[linux-mining]

Already recovered the password in less than a minute.

Kimci86 I owe you a big thanks. I think this is a great addition to an already great project.