Allow using plain lyrics

Allow the option to use plain lyrics if synced aren't available. They
can be fed into a LRC generator for making proper synced lyrics, and for
some people plain lyrics is better than no lyrics.

Changes the CLI parser to `clap` to make it easier to declare flags and
parse the ways someone might pass them. Keeps the existing behaviour as
default (`lyricsrs /path/to/music/dir` works and skips plain lyrics) and
adds the `--allow-plain` CLI flag to tell `lyricsrs` to write plain
lyrics if that's all that's available.

Cargo.toml

@@ -9,10 +9,14 @@ name = "lyricsrs"
 # See more keys and their definitions at https://doc.rust-lang.org/cargo/reference/manifest.html
 
 [dependencies]
+clap = { version = "4.5.9", features = ["derive"] }
 lofty = "0.20.1"
 reqwest = { version = "0.12.5", features = ["json", "blocking"] }
 serde = { version = "1.0", features = ["derive"] }
 serde_json = "1.0"
 tokio = { version = "1", features = ["full"] }
 urlencoding = "2.1.3"
 walkdir = "2.5.0"
+
+[dev-dependencies]
+temp-dir = "0.1.13"