Husband and wife shall be competent witnesses for or against each other in all cases, civil or criminal, except that neither shall be allowed to testify against the other as to a statement, conversation, letter, or other communication made to the other or to another person, nor shall either be allowed in any case to testify as to a matter which, in the opinion of the court, would lead to a violation of marital confidence. This section shall not be construed so as to prevent a libelant and libelee from testifying as to all matters in divorce causes.