Make femme dependency optional

[rossmacarthur]

Description

Make the femme dependency optional, enabled via the logger feature.

Motivation and Context

Allows you to use a different log crate and femme doesn't have to be installed.

Types of changes

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to change)
  Technically speaking this is a breaking change in the case where someone removed the logger feature and still used tide::log::LevelFilter.

Checklist:

• My change requires a change to the documentation.
  • I have updated the documentation accordingly.
• I have read the CONTRIBUTING document.
• I have added tests to cover my changes.
• All new and existing tests passed.

[Fishrock123]

What's this for?

[rossmacarthur]

Currently the dependency tree looks like this

tide
   -- kv-log-macro
      -- log (features = ["kv_unstable"])
   -- femme
      -- log (features = ["kv_unstable", "std"])

However, tide actually relies on the std feature in the log crate being turned on. Otherwise you get the following error

 the trait `log::kv::value::ToValue` is not implemented for `std::string::String`

Ideal way to solve this is actually like this: yoshuawuyts/kv-log-macro#7

[Fishrock123]

merging as is for now, yosh will have to address the kv-log-macro thing some other time.