feature: add option to use custom corpus file

[dune73]

With the URL encoding issue on 920220/920221 PL1 (coreruleset/coreruleset#3926) we have a file that has a corpus that brings an FP with every payload. Running ftw with this very corpus would be useful.

Stretch goal: support corpus files without leading [0-9]+\ TAB on every line.

[fzipi]

Yes, we have thought about this already for using a File corpus

[dune73]

Custom Corpus example file in original corpus format with line number and tab.
custom-corpus.txt