add `drop_na_cols` parameter to `format_sumstats()`

[rmgpanw]

This PR aims to address issue #181. Proposed changes for review:

• Update format_sumstats() and check_miss_data() with a new drop_na_cols parameter
• Add simple validation checks for drop_na_cols in validate_parameters()
• Added a couple of new tests in test-missing_data.R

Many thanks in advance for reviewing and considering

[Al-Murphy]

Looks good thanks for doing this! Will merge and add some small changes and add you as a contributor

[Al-Murphy]

@rmgpanw Can I get your name to add you to the contributors of the package README?

[rmgpanw]

That's brilliant! My name is Alasdair Warwick, thanks :)