Check that embedded MD5 exists before verifying or embedding

[MarcosSueiro]

I could not tell from the code (just from the time it takes to implement the function), but it seems BWFME calculates an MD5 before embedding it with --MD5-embed. This makes the process potentially much slower, especially when implementing these functions on a large set of files that includes a subset of files with embedded MD5 data.

A logical sequence for both --MD5-Verify and --MD5-embed (although perhaps not so for --MD5-embed-overwrite) might be:

1. Read the file to see if MD5 data exists in the file
2. If yes to (1), calculate the MD5
3. (Verify) or (Embed)