Do not set the terminal to raw mode if it isn't actually a terminal

It doesn't make sense to try to set a terminal as a raw terminal if it
isn't actually a terminal. Worse, this may break tests that assume they
can simply provide a pipe to a subprocess and expect the terminal will
be unchanged.

See this PR, which needs this patch:
sharkdp/bat#2631

src/lib.rs

@@ -1,6 +1,6 @@
 use crossterm::terminal;
 use std::env;
-use std::io::{self, Write};
+use std::io::{self, Write, IsTerminal};
 use std::time::{Duration, Instant};
 use thiserror::Error;
 #[cfg(target_os = "windows")]
@@ -179,6 +179,10 @@ pub fn theme(timeout: Duration) -> Result<Theme, Error> {
 }
 
 fn from_xterm(term: Terminal, timeout: Duration) -> Result<Rgb, Error> {
+    if !std::io::stdout().is_terminal() {
+        return Err(Error::Unsupported);
+    }
+
     // Query by XTerm control sequence
     let query = if term == Terminal::Tmux {
         "\x1bPtmux;\x1b\x1b]11;?\x07\x1b\\\x03"