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

	My Desktop	My Laptop
Age	6 years	9 years
CPU	i5 3570K	Turion 64 X2 Mobile
Storage	SSD	5400 RPM HDD
Latency	0.12s - 0.14s	0.30s - 0.40s

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)