Support handler located at subfolder

[felipeespitalher]

I'm facing the same mentioned at this issue #136.
So, I made some changes to validate if "wsgi_handler.handler" is at subfolder.

Here is an example how I would like/need to use this plugin

functions:
  api:
    handler: api/wsgi_handler.handler

[codecov]

Codecov Report

Merging #198 (3f4cf44) into master (0e5789b) will not change coverage.
The diff coverage is 100.00%.

@@           Coverage Diff           @@
##           master     #198   +/-   ##
=======================================
  Coverage   99.31%   99.31%           
=======================================
  Files           1        1           
  Lines         292      292           
  Branches       73       73           
=======================================
  Hits          290      290           
  Misses          2        2           

Impacted Files	Coverage Δ
index.js	99.31% <100.00%> (ø)

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[logandk]

@felipeespitalher Thanks!