feat: add skip option

resolves #16 and #87

src/input.rs

@@ -0,0 +1,38 @@
+use std::cell::RefCell;
+use std::fs;
+use std::io::{self, Read, Seek, SeekFrom};
+
+pub enum Input<'a> {
+    File(fs::File),
+    Stdin(RefCell<io::StdinLock<'a>>),
+}
+
+impl<'a> Read for Input<'a> {
+    fn read(&mut self, buf: &mut [u8]) -> io::Result<usize> {
+        match *self {
+            Input::File(ref mut file) => file.read(buf),
+            Input::Stdin(ref mut stdin) => stdin.borrow_mut().read(buf),
+        }
+    }
+}
+
+impl<'a> Seek for Input<'a> {
+    fn seek(&mut self, pos: SeekFrom) -> io::Result<u64> {
+        match *self {
+            Input::File(ref mut file) => file.seek(pos),
+            Input::Stdin(_) => Err(io::Error::new(
+                io::ErrorKind::Other,
+                "not supported by stdin-input",
+            )),
+        }
+    }
+}
+
+impl<'a> Input<'a> {
+    pub fn into_inner(self) -> Box<dyn Read + 'a> {
+        match self {
+            Self::File(file) => Box::new(file),
+            Self::Stdin(stdin) => Box::new(stdin.into_inner()),
+        }
+    }
+}