Add scrape of ETF top/major holdings

[dotJoel]

This PR is a a completion of the work started in #1571

To address the issue with returning price information when checking the holdings of a non-fund ticker, it does the following:

1. check for the presence of a redirect away from /holdings to identify when they are n/a.
2. return empty list in that case

[ValueRaider]

Can it be converted to a DataFrame? More usable. Use get_earnings_dates for inspiration.

• symbol as index
• convert % Assets to actual number

Documentation - update README.

Add a unit test, in case someone in future breaks your code.

[dotJoel]

It seems that this data is no longer a table on that page?
Top holdings that were a table now seem to be divs.
I swear this worked last week...

I'd rather help with the quoteSummary than parse more html.

[ValueRaider]

Does the webpage fetch from API? Firefox -> F11 -> Network -> refresh

[dotJoel]

Not directly.
There is a JS script block that has the data that comes from the API.
Seems to be the same API endpoint that the Fund data PR is using.

[ValueRaider]

btw your commit history is a mess, I can't merge that.

[ValueRaider]

I've fixed formatting but only tested on IVV. Can people try more ETFs?