Add a way to configure the format of the version string

[pwoolcoc]

This adds the ability to add a section to a .cargo/config file that
will tell cargo-edit how to format the version string when adding or
upgrading dependencies. By adding a [cargo-edit] section to a
project's or user's .cargo/config, the user can control how versions
are added to a Cargo.toml.

This will only work with exact versions, version ranges are not
supported.

Here is an example that will cause cargo-edit to only print the major
and minor version number for a dependency:

# .cargo/config
[cargo-edit]
version_fmt = "{MAJOR}.{MINOR}"

You can also have separate formats for adding vs upgrading:

# .cargo/config
[cargo-edit.add]
version_fmt = "{MAJOR}.{MINOR}.{PATCH}"

[cargo-edit.upgrade]
version_fmt = "{MAJOR}"

Closes #126

[shikhar]

We would like to have predictable lock-files with Bazel rules_rust (which can inter-operate with Cargo so still use this plugin for upgrading dependencies). This requires specifying exact versions ={MAJOR}.{MINOR}.{PATCH}, so this kind of templating capability would be a very useful feature for us.