allow for configuration of what raises a `KeyboardInterrupt`

[Cube707]

this PR intereduces a config submodle. It allows for central managment of configurations and also for the user to change these as needed.

For now this module only containes INTERRUPT_KEYS, a list of keys that result in readchar() raising a KeyboardInterrupt. It defaults to key.CTRL_C

Testing implementation:

• [x] existing default behavior

  • [x] import full libary

    
    import readchar

  while True: k = readchar.readkey()

  
  - [x] from import
  ```python
  from readchar import readkey
  
  while True:
      k = readkey()

• [x] import full libary

  import readchar
  
  readchar.config.INTERRUPT_KEYS.append("q")
  while True:
    k = readchar.readkey()

• [x] from import

  from readchar import readkey, config
  
  config.INTERRUPT_KEYS.clear()
  config.INTERRUPT_KEYS.append("q")
  
  while True:
    k = readkey()

  from readchar import readkey, config
  
  config.INTERRUPT_KEYS = ["q"]
  
  while True:
    k = readkey()

Warning only works with the list methodes!! using config.INTERRUPT_KEYS = [] will not work

changed so that config is a class with class-attributes, this makes it much easier to interact with. Now both of these work, making it much easyer for the user:

# now works:
config.INTERRUPT_KEYS = ["q"]
# still works:
config.INTERRUPT_KEYS.clear()
config.INTERRUPT_KEYS.append("q")

[coveralls]

Coverage increased (+0.3%) to 94.737% when pulling d3f69913f277ac6d0c6c51424c48fb5fa33afbc1 on config-interrupt-keys into b7d554ce08e5352585917c75c62586606712db07 on master.