Add toml as an optional dep/feature

also expand the test matrix so we test with

* zero features
* each feature individually
* all features together

.travis.yml

@@ -8,7 +8,9 @@ before_script:
 script:
   - bash -c 'if [[ "$TRAVIS_RUST_VERSION" == "nightly"* ]]; then cargo fmt --all -- --check; fi'
   - cargo build
-  - cargo test
+  - cargo build --features toml
+  - cargo build --features all
+  - cargo test -- features all
 matrix:
   allow_failures:
     - rust: nightly

Cargo.toml

@@ -18,6 +18,7 @@ serde_derive = "1"
 serde_json = "1"
 url = "1"
 try_from = "0.2.2"
+toml = { version = "0.4.6", optional = true }
 
 [dependencies.chrono]
 version = "0.4"
@@ -25,3 +26,10 @@ features = ["serde"]
 
 [dev-dependencies]
 toml = "0.4"
+
+[features]
+default-features = []
+all = ["toml"]
+
+[package.metadata.docs.rs]
+features = ["all"]