Attach to a running process from command line.

**Describe the bug**
I am not sure if this can be treated as a bug. But it would be great to have the capability to
attach to a running process from the command line.
eg : gdbgui --attach_process 3434
where 3434 is the PID of the process.

**To Reproduce**
Steps to reproduce the behavior:
We do not have a way to do so.

**Expected behavior**
Launch browser with the process attached.

**Screenshots**
None.

**Please complete the following information:**
* OS:
* gdbgui version (`gdbgui -v`):
* gdb version (`gdb -v`):
* browser [e.g. chrome, safari]:
* python packages (`pip freeze`):

**Additional context**
Add any other context about the problem here.

Start at the gdbgui command-line entry point and trace how a browser session currently launches or selects a process. Use the proposed `gdbgui --attach_process 3434` invocation as the acceptance check: the browser should open with the specified PID attached.

Written by the indexing model from the issue text.