Bugfix: Fix postprocessInput and add tests

[nalinbhardwaj]

PR Checklist

• [x] I have read the Contributing Guidelines
• [x] New functionality (or bug fix) is appropriately tested
• [x] PR includes closes #{issue number} if appropriate

  Fixes the way post processing of input works so it doesn't add name of unit to all elements of numbers, just the last one. Fixes formatConversion() to retain joiner if it exists before.

  See added tests for examples of conversions and their formats.

  Relevant link(s):

  Closes #111

[coveralls]

Coverage increased (+0.2%) to 90.92% when pulling a37453fe913e6c10483373e81c276d843a10c5a8 on nalinbhardwaj:bug/joiners into 934421ef9b15693b118c149f7046c3878f48407c on cannawen:master.