Make rate limiting message configurable

[dsaxton]

Just a humble PR to make the rate-limiting message configurable.

Testing:

Edit messages under [filters.ratelimit] in noteguard.toml then

$ make build-linux
...
$ ./test/delayed | ./target/x86_64-unknown-linux-musl/release/noteguard
{"id":"68421a122cef086512b2c5bd29ca6285ced8bd8e302e347e3c5d90466c860a76","action":"accept"}
{"id":"68421a122cef086512b2c5bd29ca6285ced8bd8e302e347e3c5d90466c860a76","action":"accept"}
{"id":"68421a122cef086512b2c5bd29ca6285ced8bd8e302e347e3c5d90466c860a76","action":"accept"}
{"id":"68421a122cef086512b2c5bd29ca6285ced8bd8e302e347e3c5d90466c860a76","action":"accept"}
{"id":"68421a122cef086512b2c5bd29ca6285ced8bd8e302e347e3c5d90466c860a76","action":"accept"}
{"id":"68421a122cef086512b2c5bd29ca6285ced8bd8e302e347e3c5d90466c860a76","action":"accept"}
{"id":"68421a122cef086512b2c5bd29ca6285ced8bd8e302e347e3c5d90466c860a76","action":"accept"}
{"id":"68421a122cef086512b2c5bd29ca6285ced8bd8e302e347e3c5d90466c860a76","action":"accept"}
{"id":"68421a122cef086512b2c5bd29ca6285ced8bd8e302e347e3c5d90466c860a76","action":"reject","msg":"this is my custom message"}
{"id":"68421a122cef086512b2c5bd29ca6285ced8bd8e302e347e3c5d90466c860a76","action":"reject","msg":"this is my custom message"}
{"id":"68421a122cef086512b2c5bd29ca6285ced8bd8e302e347e3c5d90466c860a76","action":"reject","msg":"this is my custom message"}
{"id":"68421a122cef086512b2c5bd29ca6285ced8bd8e302e347e3c5d90466c860a76","action":"reject","msg":"this is my custom message"}
{"id":"68421a122cef086512b2c5bd29ca6285ced8bd8e302e347e3c5d90466c860a76","action":"reject","msg":"this is my custom message"}
...

[jb55]

we probably want Option<String> here

[jb55]

thanks! squashed into 77c2a89,

made a small optimization in 778c5a0, separated your makefile change into 3b73e9a, and documented this new string in db9ef05