Use LESS environment variables for less options

Previously, we would either use the pager set in the PAGER environment
variable or "less -cr" for the rich and plain viewer.  This caused
problems if, for example, the PAGER environment variable was set to
"less", see ticket #22:
	https://todo.sr.ht/~ireas/rusty-man/22

To fix this issue, we now use the LESS environment variable to set the
less options.  This makes sure that setting PAGER does not override the
options.

CHANGELOG.md

@@ -5,7 +5,11 @@ SPDX-License-Identifier: MIT
 
 # Changelog for rusty-man
 
-# v0.4.0 (2020-10-09)
+## Unreleased
+
+- Use the `LESS` environment variable to set the options for the pager.
+
+## v0.4.0 (2020-10-09)
 
 This minor release introduces a new interactive viewer, tui.  It also adds
 syntax highlighting for code in the documentation and support for Rust 1.47.0.

src/viewer/text/mod.rs

@@ -4,6 +4,7 @@
 mod plain;
 mod rich;
 
+use std::env;
 use std::io;
 
 use crate::args;
@@ -63,7 +64,10 @@ impl viewer::Viewer for TextViewer {
 }
 
 pub fn spawn_pager() {
-    pager::Pager::with_default_pager("less -cr").setup()
+    if env::var_os("LESS").is_none() {
+        env::set_var("LESS", "cR");
+    }
+    pager::Pager::with_default_pager("less").setup()
 }
 
 fn ignore_pipe_error(error: io::Error) -> io::Result<()> {