Allow --filename={}/file.csv in addition to {}.csv

This is a minor tweak to handle some problems which I've encountered
several times now.  The only real excuse for this change is that it's
unobstrusive and has natural semantics.

This patch modifies --filename to support two use cases:

    xsv partition --filename {}/cities.csv state . all-cities.csv
    xsv partition --filename {}/monuments.csv state . all-monuments.csv
    xsv partition --filename {}/parks.csv state . all-parks.csv

Above, we want to partition our records by state into separate
directories, but we have multiple kinds of data.

    xsv partition --filename {}/$(uuidgen).csv . input1.csv
    xsv partition --filename {}/$(uuidgen).csv . input2.csv

Above, we're running multiple (possibly parallel) copies of xsv and we
want to parition the data into multiple directories without a filename
clash.

There's one limitation to the implementation: We might theoretically hit
the `create_dir_all` race condition recently fixed by
rust-lang/rust#39799.  I'm planning to supply a final patch which works
around this race condition in stable Rust.

Author: emk

src/util.rs

@@ -176,6 +176,11 @@ impl FilenameTemplate {
     {
         let filename = self.filename(unique_value);
         let full_path = path.as_ref().join(filename);
+        if let Some(parent) = full_path.parent() {
+            // If you're likely to call this concurrently, see
+            // https://github.com/rust-lang/rust/pull/39799
+            fs::create_dir_all(parent)?;
+        }
         let spath = Some(full_path.display().to_string());
         Config::new(&spath).writer()
     }

tests/test_partition.rs

@@ -95,6 +95,22 @@ fn partition_custom_filename() {
     assert!(wrk.path("state-NY-partition.csv").exists());
 }
 
+#[test]
+fn partition_custom_filename_with_directory() {
+    let wrk = Workdir::new("partition_custom_filename_with_directory");
+    wrk.create("in.csv", data(true));
+
+    let mut cmd = wrk.command("partition");
+    cmd.args(&["--filename", "{}/cities.csv"])
+        .arg("state")
+        .arg(&wrk.path("."))
+        .arg("in.csv");
+    wrk.run(&mut cmd);
+
+    // This variation also helps with parallel partition jobs.
+    assert!(wrk.path("NY/cities.csv").exists());
+}
+
 #[test]
 fn partition_invalid_filename() {
     let wrk = Workdir::new("partition_invalid_filename");