Don't repeat default values for command line help

[gundalow]

./sp0rkle --help 2> before
# make fixes
./sp0rkle --help 2> after
diff before after
3c3
<       Comma-separated list of channels to join, defaults to '#sp0rklf' (default "#sp0rklf")

---
>       Comma-separated list of channels to join. (default "#sp0rklf")
5c5
<       Address of MongoDB server to connect to, defaults to localhost. (default "localhost")

---
>       Address of MongoDB server to connect to. (default "localhost")