Make sensitive keys set configurable

[jeffrey-effendy]

Currently the masking logic has hard coded set of sensitive keys under RequestModelFactory to mask the body value. There are times where I need to add values into the existing keys. It is hard with the current implementation.

Since the masking logic is already implemented, it would be nice to have the sensitive keys configurable from Django settings. This will make the masking more flexible for many users.

For this I create a new key in config
SILKY_SENSITIVE_KEYS

with default value
{'username', 'api', 'token', 'key', 'secret', 'password', 'signature'}
so it does not affect any previous users.

Let me know if this is a good addition

[codecov]

Codecov Report

Merging #494 (0c237b3) into master (11aa8cd) will increase coverage by 0.03%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           master     #494      +/-   ##
==========================================
+ Coverage   84.61%   84.65%   +0.03%     
==========================================
  Files          50       51       +1     
  Lines        2061     2066       +5     
==========================================
+ Hits         1744     1749       +5     
  Misses        317      317              

Impacted Files	Coverage Δ
silk/config.py	90.47% <ø> (ø)
silk/model_factory.py	83.84% <100.00%> (ø)
silk/apps.py	100.00% <0.00%> (ø)
project/project/__init__.py	66.66% <0.00%> (+6.66%)	⬆️

---

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 11aa8cd...0c237b3. Read the comment docs.