Use 'testthat' to keep our package in shape

[timtreis]

As briefly discussed in Slack, we could use testthat to perform various checks to keep our codebase in line with CRAN requirements. Additionally, if we decide to advertise our package's riskmetric score, we could also integrate checks for that.

CRAN:

• No T/F, but TRUE/FALSE
• ...

riskmetric:

• "NEWS file contains entry for current version number"
• has_vignettes
• has_bug_reports_url
• export_help
• has_website
• r_cmd_check [Broken?]
• remote_checks [Broken?]
• has_maintainer
• has_news
• has_source_control
• covr_coverage [Broken?]