Add suspend functionality to keybindings for toolong

[iconoclasthero]

To address #78, added

        Binding("ctrl+z", "suspend_process", "Suspend", key_display="^z"),

to BINDINGS section of log_view.py to enable suspending/SIGSTP/^Z functionality.
Left ^z visible; did not modify help documentation.
Alternately, could change to

        Binding("ctrl+z", "suspend_process", "Suspend", key_display="^z", show=False),

to remove from baseline display and add to help.

[iconoclasthero]

Upon further evaluation, it would probably be better to put this in ui.py

class LogScreen(Screen):

    BINDINGS = [
        Binding("f1", "help", "Help"),
        Binding("ctrl+z", "suspend_process", "Suspend", key_display="^z"),
    ]

so that it will work more universally within the textualize environment, e.g., if toolong is called with more than one log file from the cli (tl log1.log log2.log).