core: get cmd outPath removes trailing slash - fixes #3729

When using path with trailing slashes, ipfs get command had issues
with trying to store content into '.' folder (as the file part of the path
was empty). Now, it correctly stores the file into desired folder,
thanks to the removal of trailing slash by introducing a
getOutPath function in get command, which does proper parsing
Of the output path.

BEFORE:
 $  .pfs get /ipns/multiformats.io/
Saving file(s) to .

NOW:
$ ./ipfs get /ipns/multiformats.io/
Saving file(s) to multiformats.io

License: MIT
Signed-off-by: adamliesko <adamliesko@gmail.com>