Added option to search with filters

[Goncalo-Chambel]

I've created an option so that you can search for the images using the available filters from Google, for example the image size or the image color.

I have added an example in the sample.py file and in the simple_image_download.py file, there is a description of the available filters and their meaning. I did not add that information to the README however.

I have also implemented a function to make sure the filters chosen are correcty formatted and if not, a error message is shown.

Let me know if you like this addition and if there is anything else I can do

[Goncalo-Chambel]

Edit
Modified the method to split the keywords because using spaces for the search term was splitting the keywords. For example "black hole" should be considered as one search term but was being split into "black" and "hole". You can still use multiple search terms, just separate them using a comma

[Pr0mises]

Thank you! Hope it get's merged