allows users to configure the TTL of conversation info data

[aoberoi]

Summary

This PR adds configurability to the time Hubot will consider the data returned from conversations.info "fresh", without fetching it once again. This allows users to address concerns such as the on raised in #554.

Requirements (place an x in each [ ])

• I've read and understood the Contributing Guidelines and have done my best effort to follow them.
• I've read and agree to the Code of Conduct.

[codecov]

Codecov Report

Merging #560 into master will increase coverage by 0.03%.
The diff coverage is 100%.

@@            Coverage Diff             @@
##           master     #560      +/-   ##
==========================================
+ Coverage   86.29%   86.32%   +0.03%     
==========================================
  Files           6        6              
  Lines         372      373       +1     
  Branches       83       83              
==========================================
+ Hits          321      322       +1     
  Misses         29       29              
  Partials       22       22

Impacted Files	Coverage Δ
src/client.coffee	95.19% <100%> (+0.04%)	⬆️

---

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[codecov]

Codecov Report

Merging #560 into master will increase coverage by 0.03%.
The diff coverage is 100%.

@@            Coverage Diff             @@
##           master     #560      +/-   ##
==========================================
+ Coverage   86.29%   86.32%   +0.03%     
==========================================
  Files           6        6              
  Lines         372      373       +1     
  Branches       83       83              
==========================================
+ Hits          321      322       +1     
  Misses         29       29              
  Partials       22       22

Impacted Files	Coverage Δ
src/client.coffee	95.19% <100%> (+0.04%)	⬆️

---

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 e3acc28...a5d67e2. Read the comment docs.