Add tag option to bookmarks to fetch multiple bookmarks

In some cases, you may want to get all bookmarks related to a tag. You may not remember all of the names of bookmarks.

This creates another command or flag. 

```shell
$ bm add sam "/home/sam" -t homes

$ bm add adam /home/adam -t homes

$ bm show tag homes
-----------------
sam  : /home/sam
adam : /home/adam

$ bm show tag
homes
...
```