Support overrding the credentialsMatcher for the implicit IniRealm

When using the implicit IniRealm, it would be useful to override the credentialsMatcher to support, non-plaintext passwords in the [users] section.

Example shiro.ini:
—
[main]

1. The [users] section will contain SHA256 hashed passwords
   credentialsMatcher=org.apache.shiro.authc.credential.Sha256CredentialsMatcher

[users]

1. Hex value of SHA256 hashed password
   aUser=28eeeb5e16f38c67dfcc73733ee3649f9b0adcc2a6cfc957d40814348459ee7c
   —