Add ability to wait (with timeout) for the (gdb) "prompt".

**Is your feature request related to a problem? Please describe.**
I am looking into enhancing one of my project that is using a self written way to talk to gdb/mi and in that project I read some data with single commands and the result comes in chunks over tens of seconds. In my code I normally wait for the (gdb) string, which indicates that gdb is done writing the data, however trying pygdbmi the read functionality (_get_responses_unix) seems to entirely be based on timeouts.

**Describe the solution you'd like**
Add an option ( or make it the default ) to wait for the (gdb) prompt to indicate a commands output completion. Ideally one would have two timeouts: one for the whole read operation of all output, and one for the time after the last data is read.

**Describe alternatives you've considered**
Getting the data myself and just using pygdbmi as a parser, but thats probably not as neat.

**Additional context**
Add any other context or screenshots about the feature request here.