Pass context when calling Postmark API

[byamba-tumurkhuu]

@keighl I added client methods that accept context.Context as a parameter. This is helpful when you want to have chained http transports that use the original request context to do things like tracing and logging.

[codecov-commenter]

Codecov Report

Merging #26 into master will increase coverage by 0.37%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           master      #26      +/-   ##
==========================================
+ Coverage   97.56%   97.93%   +0.37%     
==========================================
  Files          10       10              
  Lines         410      484      +74     
==========================================
+ Hits          400      474      +74     
  Misses          5        5              
  Partials        5        5              

Impacted Files	Coverage Δ
bounce.go	97.36% <100.00%> (+0.49%)	⬆️
email.go	100.00% <100.00%> (ø)
messages_inbound.go	100.00% <100.00%> (ø)
messages_outbound.go	100.00% <100.00%> (ø)
postmark.go	76.47% <100.00%> (ø)
sender_signatures.go	100.00% <100.00%> (ø)
server.go	100.00% <100.00%> (ø)
servers.go	100.00% <100.00%> (ø)
stats.go	100.00% <100.00%> (ø)
templates.go	100.00% <100.00%> (ø)

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[byamba-tumurkhuu]

Bump @keighl