Add a section on string manipulation

[chloepugh]

The material for this section should be added to 'rmd_files/strings.Rmd'.

Include examples of some of the more common stringr() functions, such as:

• str_length()
• str_c()
• str_sub()
• str_detect()
• str_replace() ?

Also include a quick introduction to basic regex when describing the str_detect() function.