Default FileExporter file_mode to a

[gjoseph92]

The current default, w+, overwrites the contents of the file on the first write. Since emit opens and writes to the file every time it's called, this effectively rewrites the file on every emit. Perhaps this is intended behavior, but it took me a while to understand why there was only one span written to my file, even though I could tell multiple were being emitted. I didn't have my Python file mode tables memorized, so at first glance w+ sounded like "write mode, + for append", and I overlooked it while debugging.

I think a (append) would be a more intuitive default.

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