Add a cache for lexers.Get

Add a cache for lexers.Get

```
name                            old time/op    new time/op    delta
Codeblocks/Default-10              152ms ±11%      12ms ± 1%  -92.44%  (p=0.029 n=4+4)
Codeblocks/Hook_no_higlight-10     142ms ± 0%       7ms ± 0%  -95.36%  (p=0.029 n=4+4)

name                            old alloc/op   new alloc/op   delta
Codeblocks/Default-10             11.9MB ± 0%    11.7MB ± 0%   -1.59%  (p=0.029 n=4+4)
Codeblocks/Hook_no_higlight-10    4.62MB ± 1%    4.43MB ± 0%   -4.08%  (p=0.029 n=4+4)

name                            old allocs/op  new allocs/op  delta
Codeblocks/Default-10               209k ± 0%      209k ± 0%   -0.03%  (p=0.029 n=4+4)
Codeblocks/Hook_no_higlight-10     68.4k ± 0%     68.3k ± 0%   -0.06%  (p=0.029 n=4+4)

```