Do not use replaceCID if not necessary

[meain]

This PR makes two main changes:

• If the user provides a CID instead of a name, there is not need to generate a new CID
• If there are no CIDs to replace, completely avoid calling replaceCID (also avoids few additional allocations)

Follow up to https://github.com/xhit/go-simple-mail/pull/103

I was trying to speed up the .eml generation and found replaceCID to be a huge bottle neck. Here is a flamegraph of the cpu/execution time. You can see that a major chunk is taken up by replaceCID regexp search.