Adds file_browser.html example (update for row_headers feature)

[texodus]

Updates #19 from @telamonian to rely on the new row_headers feature introduced in #24 . The add-filebrowser-example branch was first rebased:

• Commit history was squashed to 1 commit
• Changes to tree_row_header.js and 'events.js` were removed.
• Changes to perspective.html were removed (which relied on the above).
• README.md changes reverted (no need to add your example to the gallery - I'll do this automatically via script when I dist).

Next, the example was refactored. Unfortunately some of my comments from the original PR were quickly outdated by rapid progress on this API.

• Removes __config, __collapse and __expand pass-through.
• Uses row_headers instead of row_indices, et al columns.
• Adds a custom tree_header renderer ala perspective.html, modified to handle the lack of an empty 'root' node.
• Adds a click handler to dispatch to collapse and expand()
• Refactors code into 2 script sections - "fake filesystem generator" and "data model for regular-table" sections.
• General cleanup and lint-powered reformat.

[telamonian]

LGTM!

The only other thing I'd like to get in as part of this PR is a way to set a text title on the names column (ie the row_headers column). You can sort of do it via row_pivots, but a) that's obviously not what it's for and b) in that case the column name still isn't visible. It's not a blocker, though

[telamonian]

This is a fix for the issue where the <span class="pd-row-header-icon"> elements were throwing off the alignment of the dir/file names, right? Nice work

[texodus]

Yes - Perspective has no way to generate datasets with "folders" at the same level as "leaves", so this number was off a few pixels.

[texodus]

Thanks for the review! I will think about concise ways to name the "corner" as well, PRs welcome.