Add sha256 support with old md5 support

[Shiranuit]

What does this PR do ?

The goal of this PR is to avoid using MD5 hashes in hexadecimal format as a key for aes-256-cbc because even if they are 32 bytes long, which should give us 256 bits password entropy (256^32 == 2^256) which is good at the first look.
We know that there would only be hexadecimal characters within the 32 bytes so it reduces the password entropy to 128 bits (16^32 == 2^128).
So we use a 256 bits password to encrypt data but we only have a 128 bits for the password entropy.

This is why this PR use SHA256 instead of MD5, to provide 256 bits length password and 256 bits of password entropy, and also support the decryption of old data encrypted with MD5 hashed keys.

[Aschen]

More an enhancement to me

[Aschen]

Please add an unit test with an AES hash

[codecov]

Codecov Report

Merging #3 into 1-dev will increase coverage by 0.58%.
The diff coverage is 100%.

@@            Coverage Diff            @@
##            1-dev      #3      +/-   ##
=========================================
+ Coverage   90.32%   90.9%   +0.58%     
=========================================
  Files           1       1              
  Lines          62      66       +4     
=========================================
+ Hits           56      60       +4     
  Misses          6       6

Impacted Files	Coverage Δ
lib/vault.js	90.9% <100%> (+0.58%)	⬆️

---

Continue to review full report at Codecov.

Legend - Click here to learn more
Δ = absolute <relative> (impact), ø = not affected, ? = missing data
Powered by Codecov. Last update 7cad38d...8828ae5. Read the comment docs.