Flatten-Tool: Expose CLI options

[jpmckinney]

For example, we can add form elements that directly match Flatten Tool's CLI options.

Alternately (or, in addition), users might not understand all the options, but can still recognize the output they want. For such users, we can consider offering a few different output formats that match common user demands (e.g. translated column headings, with only columns and sheets that contain data). This might look like a simplified form, where instead of exposing Flatten Tool's CLI options through a form, we're interpreting those options into more sensible combinations for the user.

In that case, we can still provide direct access to the CLI options in, e.g. an 'advanced' mode.

[jpmckinney]

Also: --preserve-fields templates, with common sets of desirable fields. This might be a two-step process so users can select which fields to preserve among those that are actually in the data.

[jpmckinney]

Closed by #108