Support reading from more general inputs

[jennybc]

Right now readxl reads only from xlsx and xls files.

In the fullness of time, some functionality for supporting more general inputs will be pulled out of readr, at which point readxl can exploit that. We might implement an interim solution for some of these in the meantime.

This issue will cover all related feature requests (all of which I'm now closing):

• input via rawConnection #192 input via rawConnection
• URL support? #183 URL support?
• feature request: align all read_* decompression and download behavior with read_delim()? #115 align all read_* decompression and download behavior with read_delim()
• Read file directly from memory #93 Read file directly from memory
• Feature Request: ability to read from HDFS #518 Feature Request: ability to read from HDFS

[jjcad]

Question: Along with this support for more general inputs, will that also signal a transition to readr style col_types? Having that kind of unity between the packages would be welcome.

[jennybc]

The col_types transition is #198. I can't think of any reason that implementation of these two things would be inextricably tied together. They'd more likely happen around the same time because it meant I was working on a new release.

[MichaelChirico]

fread's approach is to download the file & proceed to use plaid-old fread on that.

@jennybc If you're happy with that logic, I'd be happy to file a PR.

[jennybc]

@MichaelChirico Yeah, that is an option we've contemplated re: "faking" read from a URL. I'm definitely interested in a PR that downloads to a temp file, then reads that.