This commit updates the CLI arguments to accept either a file containing
a json pointer expression, or an expression as a commandline argument:

    usage: jsonpointer [-h] (-f [POINTER_FILE] | -p [POINTER]) [--indent INDENT] [-v] FILE [FILE ...]

Without this commit

    jsonpointer "/a" a.json b.json

works fine, however using a file doesn't work:

    $ jsonpointer -f ptr.json a.json b.json
    usage: jsonpointer [-h] [-f [POINTER_FILE]] [--indent INDENT] [-v] [POINTER] FILE [FILE ...]
    jsonpointer: error: argument POINTER: not allowed with argument -f/--pointer-file

This commit also improves the usage message by explaining the mutually exclusive arguments,
adds tests for the changes, updates the documentation and adds a new CHANGELOG.md file.

Resolves stefankoegl#43