Add Pager helper with info about where the value comes from

In preparation of fixing issue #1063.
This is a pure refactoring with no intended functional side effects.

src/pager.rs

@@ -0,0 +1,45 @@
+#[derive(Debug, PartialEq)]
+pub enum PagerSource {
+    /// From the env var BAT_PAGER
+    BatPagerEnvVar,
+
+    /// From the env var PAGER
+    PagerEnvVar,
+
+    /// From --config
+    Config,
+
+    /// No pager was specified, default is used
+    Default,
+}
+
+pub struct Pager {
+    pub pager: String,
+    pub source: PagerSource,
+}
+
+impl Pager {
+    fn new(
+        pager: &str,
+        source: PagerSource
+    ) -> Pager {
+        Pager {
+            pager: String::from(pager),
+            source,
+        }
+    }
+}
+
+pub fn get_pager(
+    pager_from_config: Option<&str>,
+) -> Pager {
+    if pager_from_config.is_some() {
+        return Pager::new(pager_from_config.unwrap(), PagerSource::Config);
+    } else {
+        return match (std::env::var("BAT_PAGER"), std::env::var("PAGER")) {
+            (Ok(bat_pager), _) => Pager::new(&bat_pager, PagerSource::BatPagerEnvVar),
+            (_, Ok(pager)) => Pager::new(&pager, PagerSource::PagerEnvVar),
+            _ => Pager::new("less", PagerSource::Default),
+        };
+    }
+}