Add a few stringr functions

[mjkarlsen]

I still need to add a few more functions so I won't close out the open issue.

Adding:

• "str_count"
• "str_detect"
• "str_length"
• "str_remove_all"
• "str_remove"
• "str_replace_all"
• "str_replace"
• "str_sub"
• "str_to_lower"
• "str_to_upper"

[markfairbanks]

See #89