look-ahead-items

I was repeating, and sometimes I wanted to look ahead.
Shifted by chunks instead of one.
私は繰り返していました、そして時々私は先を見たいと思いました。
1つではなくチャンクでシフトしました。

Examples

```rust extern crate lookaheaditems;

use lookaheaditems::ItemsBuilder;

fn main() { let raw_text = "123abcあいう一二三 456defgえおか四五六";

let items = ItemsBuilder::default()
    .set_look_ahead_size(4)
    .read(&raw_text.chars().collect())
    .build();

for look_ahead_items in items {
    println!(
        "* [{}][{}][{}][{}][{}]",
        look_ahead_items,
        if let Some(ch) = look_ahead_items.get(0) {
            ch.to_string()
        } else {
            "".to_string()
        },
        if let Some(ch) = look_ahead_items.get(1) {
            ch.to_string()
        } else {
            "".to_string()
        },
        if let Some(ch) = look_ahead_items.get(2) {
            ch.to_string()
        } else {
            "".to_string()
        },
        if let Some(ch) = look_ahead_items.get(3) {
            ch.to_string()
        } else {
            "".to_string()
        }
    );
}

} ```

Run

shell cargo run --example example

Output

plain * [123a][1][2][3][a] * [23ab][2][3][a][b] * [3abc][3][a][b][c] * [abcあ][a][b][c][あ] * [bcあい][b][c][あ][い] * [cあいう][c][あ][い][う] * [あいう一][あ][い][う][一] * [いう一二][い][う][一][二] * [う一二三][う][一][二][三] * [一二三 ][一][二][三][ ] * [二三 4][二][三][ ][4] * [三 45][三][ ][4][5] * [ 456][ ][4][5][6] * [456d][4][5][6][d] * [56de][5][6][d][e] * [6def][6][d][e][f] * [defg][d][e][f][g] * [efgえ][e][f][g][え] * [fgえお][f][g][え][お] * [gえおか][g][え][お][か] * [えおか四][え][お][か][四] * [おか四五][お][か][四][五] * [か四五六][か][四][五][六] * [四五六][四][五][六][] * [五六][五][六][][] * [六][六][][][]