Refresh tinyformat.h against upstream to quiet -Wuninitialized with C++11 on clang

[coatless]

As part of #667, I am going to need to refresh tinyformat.h from upstream. The embedded version in Rcpp last received love on Aug 29, 2015.

The proposed changes are committed into that branch under id: ec16932

The snapshot of the upstream repository is available here:
https://github.com/c42f/tinyformat/blob/3a33bbf65442432277eee079e83d3e8fac51730c/tinyformat.h

The reason for the update now is I need a fix released in March 2016 to quiet a [-Wuninitialized] complaint that arises with clang.

Once this is updated, I should be able to move forward with allowing variadic templates for the advanced exception class under C++11.

[eddelbuettel]

Sounds good. But doesn't have to be part of #667, may as well be a pre-condition for getting it done right.

[eddelbuettel]

To be more plain: I am not sure I genuinely like the fact that you enlarged a PR that already covered three dozen files by adding yet another considerable chunk.

[coatless]

@eddelbuettel ~19 files were changed and 2 were added. The rest are from the remote merge. After I check in the latest commit, I'm tempted to just kill off that PR and rebuild from a fresh branch.

[eddelbuettel]

Ack on 19+2. And yes, we pulled a few rugs under you. If you can master a clean rebase, let's do that. Else start fresh.

[eddelbuettel]

May still be preferable to have this committed by itself as it could impact existing code.