env: add support for new '--file' option (includes testing)

.# Discussion

This commit adds support for a '-f'/'--file' option which reads "KEY=VALUE" lines from
a config (or ini) style text file and sets the corresponding environment key. This is
modeled after the same option in the `dotenv` and `godotenv` commands. Notably, this
commit does *not* add automatic loading of ".env" configuration files.

The environment variables set by reading the configuration file are set prior to any
unset (eg, `-u BAR`) or set (eg, `FOO=bar`) actions. Files are loaded in order with
later files overwriting any overlapping environment variables, then, unset actions (in
command line order) are executed, then, finally, set actions (in command line order)
are executed.

[1] [`dotenv`](https://github.com/bkeepers/dotenv)
[2] [`godotenv`](https://github.com/joho/godotenv)

tests/fixtures/env/vars.conf.txt

@@ -0,0 +1,4 @@
+# comment
+FOO=bar
+
+BAR="bamf this"

tests/test_env.rs

@@ -28,6 +28,36 @@ fn test_echo() {
     assert_eq!(out, "FOO-bar");
 }
 
+#[test]
+fn test_file_option() {
+    let out = new_ucmd!()
+        .arg("-f").arg("vars.conf.txt")
+        .run().stdout;
+
+    assert_eq!(out.lines().filter(|&line| line == "FOO=bar" || line == "BAR=bamf this").count(), 2);
+}
+
+#[test]
+fn test_combined_file_set() {
+    let out = new_ucmd!()
+        .arg("-f").arg("vars.conf.txt")
+        .arg("FOO=bar.alt")
+        .run().stdout;
+
+    assert_eq!(out.lines().filter(|&line| line == "FOO=bar.alt").count(), 1);
+}
+
+#[test]
+fn test_combined_file_set_unset() {
+    let out = new_ucmd!()
+        .arg("-u").arg("BAR")
+        .arg("-f").arg("vars.conf.txt")
+        .arg("FOO=bar.alt")
+        .run().stdout;
+
+    assert_eq!(out.lines().filter(|&line| line == "FOO=bar.alt" || line.starts_with("BAR=")).count(), 1);
+}
+
 #[test]
 fn test_single_name_value_pair() {
     let out = new_ucmd!()