Feature: Add --search option for saved conversations

[duluk]

I have added feature to my fork. I was going to create a pull request and noticed the Contributing Guidelines requested creating an issue before the PR.

The feature is simple. The --search option will look through the titles in the database and return matches. It currently searches only the titles, but in the future I may add searching the gob files.

I wrote it similarly to listConversations(), so it uses selectFromList() for TTY or printList() otherwise.

Here's what it looks like:

[caarlos0]

that's cool!