feat: allow -f option to accept a file path

[Cycatz]

I've been using reple for a while and it's been great! But I found I can't specify my local config file with -f option.
Although I've seen it was mentioned in #4, it seemed not work for me.

$ reple -f /tmp/cxx.json
Traceback (most recent call last):
  File "/home/cycatz/workspace/reple/reple/reple", line 8, in <module>
    reple.run_reple(sys.argv[1:])
  File "/home/cycatz/workspace/reple/reple/reple.py", line 283, in run_reple
    config = json.load(open(fname, 'r'))
FileNotFoundError: [Errno 2] No such file or directory: '/home/cycatz/workspace/reple/reple/configs//tmp/cxx.json'

So I slightly modified the get_config_fname function, here are the steps:

1. try to use the argument as the file path if the file exists;
2. otherwise, fallback to use it as a config name to find under reple configs directory.