[feature] option "selected_text_only", useful to avoid duplicating title in [%selected_text_only% %title%](%url%)

[timotheecour]

D20200421T183444
I have this default:
[%text% : %title%](%url%)
which works great to get all the context (selected text, page title, and url)
however it's not great for creating link when no text is selected, as it'll result in title being duplicated.

proposal:

add option selected_text_only which solves this problem:
[%selected_text_only% : %title%](%url%)
only drawback is it'll add a : in case no text is selected, but that's better than duplicating the title.

As an enhancement, this could (later...) be supported too:

[%selected_text_only text_that_occurs_conditionally%%title%](%url%)
eg:
[%selected_text_only : %%title%](%url%)
which does the right thing, all the time.