My Mini Grep

A better version of the minigrep project in the Rust Book - Chapter 12 and my first project in Rust.

Updated sections

Used iterator features instead of performing a for loop through the vector items collected from iterator.
Added abality to pass --ignore-case argument to search with case-insensitive behavior beside environment variable IGNORE_CASE.
If the file_path argument isn't passed, it now reads from the stdin and beside:
```
minigrep query poem.txt --ignore-case
```
Can be run this way as well:
```
cat poem.txt | minigrep query --ignore-case
```