Rename bootstrap directory

Renamed bootstrap directory to _bootstrap to conform to default ignore
pattern of autostow.sh, and make it visually clear that this is a
special directory, not one used for the dotfiles itself.

_bootstrap/autostow.sh

@@ -6,7 +6,7 @@
 #
 # Invokes stow for every folder within this repository, and that's it.
 
-ignore="${AUTOSTOW_IGNORED_DIRS:-"bootstrap:assets"}"
+ignore="${AUTOSTOW_IGNORED_DIRS}"
 
 main() {
   case "$1" in

install.sh

@@ -8,7 +8,7 @@
 #
 # Will symlink all my dotfiles into their correct places using autostow.sh
 
-bootstrap_dir="${BOOTSTRAP_DIRECTORY:-./bootstrap}"
+bootstrap_dir="${BOOTSTRAP_DIRECTORY:-./_bootstrap}"
 
 main() {
   local cmd=""