v2.3.1 vs v2.2.0

Features

[d2451a0] Addition of a NULL Backend to provide a simple way of disabling backend lookups (maartenvanderhoef)
[9c97bdf] Add go modules (#144) (Nick Irvine)

[23f8ddb] update depdencies to use working version of analytics-go (Rob McQueen)
[9d49cf8] Reinstate analytics (#139) (Nick Irvine)
[1e7134a] Revert "Reinstate analytics (#139)" (Rob McQueen)
[4b62d9e] Reinstate analytics (#139) (Nick Irvine)
[964b9d3] Revert "Merge pull request #136 from segmentio/systmeizer/add-analytics" (Nick Irvine)
[1365e40] Merge pull request #136 from segmentio/systmeizer/add-analytics (Rob McQueen)
[47c82aa] Adding analytics for usage metrics (Rob McQueen)

[0317aaf] Remove extra history entry for the current parameter version (#157) (Michael F Booth)
[cf28a31] Fix missing history and failed reads beyond secret version 50 (#158) (Michael F Booth)
[304fdba] hotfix for broken build introduced by #95 also gofmt (Nick Irvine)
[c82e7fc] added list sorting options (#95) (Bryce Hendrix)
[4fb5cd8] Fix 2 typos (Joseph Herlant)
[28b12a8] Quote environment variables. (#150) (Joshua Carp)
[2fd07a7] Handle errors in secret store constructors. (#135) (Joshua Carp)
[7b5f2b8] Use built-in pagination instead of loops. (#121) (Joshua Carp)
[7f09043] remove debug line (Rob McQueen)