Add get sql function

[cjrace]

Brief overview of changes

Add a get_clean_sql function to resolve #55, and details on code coverage setup to resolve #53.

This increases the package version to 0.2.0.

Why are these changes being made?

This function is used across lots of DfE projects already. Other bits of tidy up to resolve known issues with documentation.

Detailed description of changes

Updates to contributing page:

• Typo fix on usethis::use_test()
• Extra information on TDD approach and roxygen2 documentation
• Typo fix on lintr::lint_package()
• Reformat keyboard shortcuts section so it's easier to see
• Add details on test coverage checks
• Add instructions for adding a new vignette

Updates to package documentation:

• Added titles into _pkgdown.yml to group functions in reference list

Added get_clean_sql() function, documentation and tests. Includes a sample SQL script to test with.

Added vignette for connecting to SQL to give more user friendly context for get_clean_sql().

Bumping package version to 0.2.0 for this and #59.

Added note to README pointing to the reference documentation for the package.

Additional information for reviewers

1. Use the following line to preview package documentation updates locally:

devtools::build_site()

2. I will squash the commits when merging as I got into a slight mess with git rebase conflicts that seems to have duplicated commits in the commit history.

3. I'll reflect any changes here in Fix typo and update guidance to point to dfeR package analysts-guide#43 to update the relevant section of the Analysts' Guide.

Issue ticket number/s and link

#55, #53.

[cjrace]

Add SQL functions into dfeR

[rmbielby]

Mainly just a few minor tweaks to the text.