Adds support for CONSUL_HTTP_TOKEN_FILE

[MattDietz]

Adds support for CONSUL_HTTP_TOKEN_FILE, which mirrors functionality
found into the official Go client by Hashicorp. If defined, the
CONSUL_HTTP_TOKEN_FILE will override tokens passed in any other way
to the client.

[MattDietz]

For reference:

https://github.com/hashicorp/consul/blob/master/api/api.go#L34-L36

https://github.com/hashicorp/consul/blob/master/api/api.go#L666-L678

[codecov]

Codecov Report

Merging #32 (4be9cba) into master (7aa1ff4) will decrease coverage by 0.30%.
The diff coverage is 14.28%.

@@            Coverage Diff             @@
##           master      #32      +/-   ##
==========================================
- Coverage   94.45%   94.15%   -0.31%     
==========================================
  Files           6        6              
  Lines        1840     1847       +7     
==========================================
+ Hits         1738     1739       +1     
- Misses        102      108       +6     

Flag	Coverage Δ
unittests	94.15% <14.28%> (-0.31%)	⬇️

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Impacted Files	Coverage Δ
consul/base.py	93.45% <14.28%> (-0.34%)	⬇️

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