Multi-line regex support for search pattern

[achary]

First, thanks for this nice tool. I exactly understand your motivation for switching from bumpversion and I admit that solving various workflow and integration issues (like incorporating change log generation in the same transaction) became nice and easy with tbump.

Now to the issue.
I'm using the tool to support version tracking for project, where a single line match is not ambiguity-free. Eg. the version is stored as:

[[package]]
name = "my-project"
version = "1.1.0"

along with references to other thirdparty packages, let's say:

[[package]]
name = "rustc-hash"
version = "1.1.0"

[[package]]
name = "scopeguard"
version = "1.1.0"

Making a search formula in tbump.toml to bump only the my-project version string to 1.2.0 is currently not possible IMO.

I suggest supporting multi-line regex, so things like \n line ending could be added to make unique matches possible in situations like this:

search = '^name = "my-project"\nversion = "{current_version}"'

[achary]

Here is a PR for review for it: #98.