Implement Serialize and Deserialize from serde in Pattern

[kailan]

I need to load glob patterns from a configuration file but got an error as there is no Deserialize implementation for Pattern.

This change adds a serde feature to the crate that enables an implementation of serde::Serialize and serde::Deserialize on the Pattern struct.

Closes #136.

[tgross35]

I don't think we want this functionality: the usecase is niche, and we may not have the internal &str available in a future version. For the purpose of loading config, you can get what you need with serde::deserialize_with, which also lets you set the flags you want.

regex::Regex similarly does not provide serde traits.

[tgross35]

With the above in mind, I am going to close this and the issue: it is reasonably straightforward to reconstruct a glob from the deserialized string.

Thank you for the submission!

[kailan]

Yep, that makes sense. Thank you for your review!