config consul source supports slash as prefix

`config.NewConfig()` with consul source will both read from consul
and watch consul for changes. Hence, the `prefix` is used in these
2 cases:

- read case: it is used to strip path based on the `KVPair` returned
from consul `kv.List()` method
- watch case: it is used as the `key` of watch query (`keyprefix` type)

So for *watch case*, the `key` is leagal to be `/` for watching change
on root. While for *read case*, because `KVPair.Key` is always stripped
off the leading slash, so if user specified some `prefix` with leading
slash, we should strip it also.

An extream case would be: user want's to read & watch node in root dir.
One would specify `prefix` as `/`, and it should work then.

config/source/consul/README.md

@@ -4,7 +4,7 @@ The consul source reads config from consul key/values
 
 ## Consul Format
 
-The consul source expects keys under the default prefix `micro/config`
+The consul source expects keys under the default prefix `/micro/config`
 
 Values are expected to be json
 
@@ -29,8 +29,8 @@ Specify source with data
 consulSource := consul.NewSource(
 	// optionally specify consul address; default to localhost:8500
 	consul.WithAddress("10.0.0.10:8500"),
-	// optionally specify prefix; defaults to micro/config
-	consul.WithPrefix("my/prefix"),
+	// optionally specify prefix; defaults to /micro/config
+	consul.WithPrefix("/my/prefix"),
   // optionally strip the provided prefix from the keys, defaults to false
   consul.StripPrefix(true),
 )