Update to click v7.x and cleanup custom code in the process

[sirosen]

• Look at the click 7.0 changelog
  • Identify features which can eliminate customizations of ours. In particular, these are ones I know about:
    • custom metavars can maybe go
    • case sensitive choice type can go
    • custom date type can go
    • broken pipe handling has been improved, check on our excepthook
    • hidden opt type can probably be removed
    • custom completion can potentially be replaced (numerous changes)
    • changes to automatic command naming may let us use that feature
  • Identify features and changes which could cause issues or break us
    • several changes to helptext formatting could make our help pages explode a bit, look for this
• open a branch with the update, test the results
  • make sure to use each option type and option set
  • take time with this, don't rush
• add any new tests for changes in behavior which we want to check
  • e.g. check that cmd --help does not include hidden opts
  • stick to non-network tests, as we want to do the same kind of change that we did in globus-sdk
• merge to master and release as a minor version update
  • no intentional breaking behavioral changes should be made
  • avoid bundling this with other updates