Add simple vim mode

This commit adds a "vim_mode" setting (false/`0` by default) that causes
keys to be remapped in the following way by the `ScreenManager`:

+ h -> LEFT
+ j -> DOWN
+ k -> UP
+ l -> RIGHT
+ LEFT -> h (toggle help)
+ DOWN -> j (noop)
+ UP -> k (open kill menu)
+ RIGHT -> l (lsof current process)
+ K (Shift+K) -> k (open kill menu)
+ J (Shift+J) -> K (toggle show/hide kernel threads)
+ L (Shift+L) -> l (lsof current process)

I couldn't figure out where the manpage documentation is in the repo,
though I admittedly did not look particularly hard.

I believe this change would be a welcome option for heavy vim users like myself
who would like a familiar way to get around in htop.

Settings.h

@@ -22,7 +22,7 @@ typedef struct {
 
 typedef struct Settings_ {
    char* filename;
-   
+
    MeterColumnSettings columns[2];
 
    ProcessField* fields;
@@ -49,6 +49,7 @@ typedef struct Settings_ {
    bool updateProcessNames;
    bool accountGuestInCPUMeter;
    bool headerMargin;
+   bool vimMode;
 
    bool changed;
 } Settings;