add justfile with coverage creation rules

.gitignore

@@ -1,5 +1,7 @@
 target
 *.mp3
 *.webm
-/subscriptions.txt
+subscriptions.txt
-/downloaded.txt
+downloaded.txt
+coverage
+*.profraw

justfile

@@ -0,0 +1,23 @@
+coverage-init:
+  cargo install grcov
+  rustup component add llvm-tools
+
+coverage:
+  #!/usr/bin/env bash
+  set -e
+  rm -rf ./target
+  just clean
+  export RUSTFLAGS="-Zinstrument-coverage" LLVM_PROFILE_FILE="just-%p-%m.profraw"
+  cargo +nightly build
+  cargo +nightly test
+  just generate
+  just server
+
+generate:
+  grcov . --binary-path ./target/debug/ -s . -t html --branch --ignore-not-existing --ignore "./target/" -o ./coverage/
+
+server:
+  cd ./coverage && python3 -m http.server 8001
+
+clean:
+  rm -rf ./*.prof* ./coverage/