Add -r shortcut to toggle absolute/relative line numbers

[Oliver-Looney]

Hello, this is a cool project! Would like to make this change

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Add -r shortcut to toggle absolute/relative line numbers

To close #134

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>  cargo test
warning: method `clear_screen` is never used
  --> src/terminal.rs:57:8
   |
56 | pub trait Terminal: Write {
   |           -------- method in this trait
57 |     fn clear_screen(&mut self) -> Result;
   |        ^^^^^^^^^^^^
   |
   = note: `#[warn(dead_code)]` on by default

warning: `jless` (bin "jless" test) generated 1 warning
    Finished `test` profile [unoptimized + debuginfo] target(s) in 0.02s
     Running unittests src/main.rs (target/debug/deps/jless-5cfd06cee43ef823)

...

test result: ok. 68 passed; 0 failed; 0 ignored; 0 measured; 0 filtered out; finished in 0.00s

➜  cargo build
warning: methods `clear_screen`, `output`, and `clear_output` are never used
  --> src/terminal.rs:57:8
   |
56 | pub trait Terminal: Write {
   |           -------- methods in this trait
57 |     fn clear_screen(&mut self) -> Result;
   |        ^^^^^^^^^^^^
...
72 |     fn output(&self) -> &str;
   |        ^^^^^^
...
75 |     fn clear_output(&mut self);
   |        ^^^^^^^^^^^^
   |
   = note: `#[warn(dead_code)]` on by default

warning: `jless` (bin "jless") generated 1 warning
    Finished `dev` profile [unoptimized + debuginfo] target(s) in 0.02s