Adding set_target for xclip

[britiger]

Adding the method set_target for setting a "content type" of the clipboard data. So it is possible to paste HTML if you copy a web page:

import pyperclip
pyperclip.set_target('text/html')
text = pyperclip.paste()  # text will have the content of clipboard
print(text)

May it would be usefull to implement this for other clipboards

[clach04]

I just stumbled across this and #162 which does the same for Windows.

I went ahead and merged that with this into https://github.com/clach04/pyperclip/tree/html_linux_win32 although its untested, right now I've mostly needed Windows support (and I added html support into https://github.com/clach04/xerox/tree/my-main)