Add a new option TLSCertPath

[fujimotos]

Background

#107 added a basic TLS support to Go logger. However, we don't yet provide any
configurable options regarding certificates.

This turns out to be being a hardle for developers who want to set up a encrypted
transmission secured by a self-signed cert.

Solution

Add a new option TLSCertPath that takes a list of path strings. Load each path
as Certificate and make use of them to connect Fluentd.