Refactor into library and add argument parsing

Cargo.toml

@@ -10,9 +10,22 @@ repository = "https://github.com/connorskees/grass"
 authors = ["ConnorSkees <39542938+ConnorSkees@users.noreply.github.com>"]
 edition = "2018"
 
-# See more keys and their definitions at https://doc.rust-lang.org/cargo/reference/manifest.html
+[[bin]]
+name = "grass"
+path = "src/main.rs"
+required-features = ["commandline"]
+
+[lib]
+name = "grass"
+path = "src/lib.rs"
+edition = "2018"
 
 [dependencies]
+clap = { version = "2.33.0", optional = true }
+
+[features]
+default = ["commandline"]
+commandline = ["clap"]
 
 [dev-dependencies]
 # test imports