Implement bash tab completion

If argcomplete is installed, bash tab completion will be enabled for pmbootstrap's arguments.

If argcomplete is not installed, pmbootstrap continues to work as before.

Benchmarks

I didn't notice much variation depending on what was being completed, even completing a package name didn't seem to add much if any extra time.

Methodology

--- ~/.local/share/bash-completion/completions/pmbootstrap.orig
+++ ~/.local/share/bash-completion/completions/pmbootstrap
@@ -15,7 +15,7 @@
     if compopt +o nospace 2> /dev/null; then
         SUPPRESS_SPACE=1
     fi
-    COMPREPLY=( $(IFS="$IFS" \
+    time COMPREPLY=( $(IFS="$IFS" \
                   COMP_LINE="$COMP_LINE" \
                   COMP_POINT="$COMP_POINT" \
                   COMP_TYPE="$COMP_TYPE" \

Todo

• Actually implement package completion
• Measure response time and compare with zsh script
• Only suggest long options if "--" has already been typed, never suggest short options (like what git does)