Fix grep path when a grep alias exists

[BarbUk]

Description

Fix function that try to find a full path for grep.

Motivation and Context

When a grep alias exists, _bash-it-grep and _bash-it-egrep are using /usr/bin/grep by default.
This break bash-it on debian based distribution, where grep is /bin/grep.

$ type -t grep
alias

$ type grep
grep is aliased to `grep --color=auto'

$ type -p egrep || type -p grep
# Output nothing

The fix is to use type -P:

-P	force a PATH search for each NAME, even if it is an alias,
    		builtin, or function, and returns the name of the disk file
    		that would be executed
-p	returns either the name of the disk file that would be executed,
    		or nothing if `type -t NAME' would not return `file'

How Has This Been Tested?

When using bash-it on a debian system, I have this error before each prompt command

-bash: /usr/bin/grep: No such file or directory

After the change, no more error.

Types of changes

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to change)

Checklist:

• My code follows the code style of this project.
• If my change requires a change to the documentation, I have updated the documentation accordingly.
• I have read the CONTRIBUTING document.
• If I have added a new file, I also added it to clean_files.txt and formatted it using lint_clean_files.sh.
• I have added tests to cover my changes, and all the new and existing tests pass.

[NoahGorny]

Lgtm