Change constructor to expect indexes as second argument

Prefix internal structures with _ for explicitness.
Add `size` and `width` methods

Width represents the number of indexes (columns) and size represents
how many series (rows).
Add support for extracting a column from a frame

This creates and returns new Series, referencing the Datum within
each series at the specific column index as well as the corresponding
Index in the frame's Indexes. This makes creating these very
lightweight.
Add ability to toggle the visibility of an Index

This enables showing/hiding columns of data.
Clean up
Update Header to render indexes by name
Ensure options are passed from parent to child views