BUG: fix concat to work with more iterables (GH8645)
closes #8645

Enhances `pd.concat` to work with any iterable (except specifically undesired ones like pandas objects and strings). A new test is included covering tuples, lists, generator expressions, deques, and custom sequences, and all existing tests still pass. Finally, a "what's new" entry is included (not sure this last is correct - but [pull requests guidelines](https://github.com/pydata/pandas/blob/master/CONTRIBUTING.md) mentioned it)

@waveform80 minor comment
otherwise excellent pr!

lmk after you add that test and its green
 Updated the test to include `Iterable` (I've pulled it in from `collections` rather than `collections.abc` for compatibility with 2.x and 3.2); I also moved the import into the test itself (seems more sensible given it's the only place `deque` and `Iterable` are used). Tests still pass on my machine here - hopefully Travis'll confirm the same in a bit!
 Done and done (common code in "what's new" pulled out into new section above, and bug reference moved to top of the new test)
 perfect - ping on green

wish all PRs were this easy to review!

pick some more issues!
 ping :)
 see easy peasy :)

thanks for the effort!
