Add support for rtf format

[kbenoit]

Add support for rtf files. Solves #90.

@jeroen the striprtf package seems to handle encoding issues more robustly than unrtf so I went with that.

library("readtext")
library("quanteda")
## Package version: 1.4.3

readtext("https://jeroen.github.io/files/sample.rtf") %>%
  texts() %>%
  cat()
## It is an example test rtf-file to RTF2XML bean for testing
## 
## Font size 10, plain text;
## Font size 12, bold text. Underline,bold text.
##  Underline,italic,bold text. 
## Font size 22, plain text.
##                                                  Bold text.
##                 Italic text.
## 
##    Simple table :
## 
## 
## *| 1st column | 2nd column | 3rd column | 4th column | 5th column | 
## *| 1.1 item | 1.2 item | 1.3 item | 1.4 item | 1.5 item | 
## *| 2.1 item | 2.2 item | 2.3 item | 2.4 item | 2.5 item | 
## *| 3.1 item | 3.2 item | 3.3 item | 3.4 item | 3.5 item | 
## *| 4.1 item | 4.2 item | 4.3 item | 4.4 item | 4.5 item | 
## *| 5.1 item | 5.2 item | 5.3 item | 5.4 item | 5.5 item | 
## *| Empty  | 
## *| …
## *|  | 
## *| …
## *|  | 
## *| …
## *|  | Empty | 
## *| Last items | 
## *| …
## *|  | 
## *| …
## *|  | 
## *| …
## *|  | Last items | 
## 
## 
## List :
## 
## It is the 1st row of the list
## It is the 2nd row of the list
## …
## 
## …
## 
## …
## 
## It is the last row of the list
## 
##  Here is a brief Courier text.
##   Here is a brief MS Sans - Serif text.
##   Here is a brief MS Serif text.
##   Here is a brief Times New Roman text.
## 
##   
## 
##  Some paragraphs :
## 
## Align left :
## 
##      The text you are reading is aligned left. It is an align – left text. It is also an align – left sentence.           
##         
## Align right:
## 
##   The text you are reading is aligned right. It is an align – right text. It is also an align – right sentence. 
## 
## Align centered:
## 
##         The text you are reading is aligned center. It is an align – centered text. It is also an align – centered sentence. 
## 
## Align justified:
## 
##           The text you are reading is aligned justify. It is an align – justified text. It is also an align – justified sentence.
## 
## Here are some special characters: 
## ö
## t 
## á
## rv
## í
## zt
## û
## r
## õ
##  
## ü
## tvef
## ú
## r
## ó
## g
## é
## p, which means “five flood resistant hammer drills” () in Hungarian.
## 
##    At last you can see an image :