file-crawler 0.1.5

# A fast, concurrent, customisable, optionally async and fully documented file crawler!
Features:
- fully multithreaded by default (**more than 2 times faster**, depending)
- Two options:
  - async
  - non-async
- set maximum traversal depth
- set file / folder regex
- lazyness (see lazy evaluation)
- execution a closure for every file (in the defined scope)
- having persistent data across closure invocations
- (set start directory)

## Example usage
````rust
fn main() -> Box<dyn Error> {
    use file_crawler::prelude::*;

    use std::fs::File;
    use std::path::PathBuf;
    use std::sync::Arc;
    use std::sync::atomic::AtomicU32;

    let a_count =
        Crawler::new()
            .start_dir("C\\user\\foo")
            .context(AtomicU32::new(0))
            .run(|ctx: Arc<AtomicU32>, path: PathBuf| {
                let mut contents = String::new();
                let file = File::open(path)?;
                //NOTE: this can cause an error for files not readable as UTF-8
                //which returns an error and therefore terminates the crawler
                file.read_to_string(&mut contents)?;
                contents.chars().for_each(|char| if char == 'a' { ctx.fetch_add(1); });
                Ok(())
            })?;
    println!("Appearance of the letter 'a' in \"C\\user\\foo\": {}", a_count);
}
````