Blueprint: DateRangeInput improvements

[cmslewis]

Would like to fix these remaining issues on DateRangeInput:

P1

• [x] (e.g.) When selected range is [<startDate>, <endDate>] and end field is focused, clicking on <startDate> in calendar should _clear_ the start date and _leave_ the end date in place. (Currently, it clears the selected range and then sets the clicked date as a new end date.) Should work symmetrically for start field too. (See #856)

P2

• [x] Add allowSingleDayRange prop (See #861)
• [x] Add selectAllOnFocus prop (See #858)
• [x] (Refactor) Pull date-range state-change logic into its own DateRangeSelectionStrategy class (See #880)

P3

• [x] Show min/max dates as placeholder on focus in an empty field (See #874)
• [ ] More robust controlled mode (how should we handle hover events, if at all?)
• [ ] When focused in an input field, allow keyboard incrementing of each of the date, month, or year in isolation (similar to TimeInput).

Maybe

• [x] Invoke all callbacks from startInputProps and endInputProps appropriately (e.g. onFocus, onBlur)

[cmslewis]

@giladgray @llorca I'm gonna squeeze this in for 1.13 to get it over with.

[llorca]

@cmslewis I'd add https://github.com/palantir/blueprint/issues/853 as a P1 -- it looks like every time you interact with the shortcuts, it then prevents any calendar navigation. What do you think about that one?

[cmslewis]

@llorca just fixed issue #853 in PR #857. take a look.

[llorca]

This is pretty much done. I filed https://github.com/palantir/blueprint/issues/995 and https://github.com/palantir/blueprint/issues/996 to track the couple of P3s remaining 👍