Deriving output format from file extension in "-o" target

[philrz]

In absence of an explicit -f, super could derive the output format from a file extension in a -o target.

Details

At the time this issue is being opened, super is at commit 36cb48a.

$ super -version
Version: v0.3.0-389-g36cb48a1d

@mccanne recently pointed out this hazard:

$ echo '{foo: "bar"}' | super -o foo.parquet -

The file extension in the command line clearly expresses a user's intent to create Parquet output, but since they didn't include the explicit -f parquet the contents of foo.parquet actually ended up being CSUP.