Composer plugin

- add function "composer"
- uses local composer if available
- add function "composer_install" to install composer to current dir

plugins/composer/composer.fish

+# composer command
+function composer
+  if test -e ./composer.phar # if there is composer in this path use local one
+    ./composer.phar $argv;
+  else if test -n "$COMPOSER_BIN"
+    eval "$COMPOSER_BIN" $argv;
+  else
+    echo "Please install composer.phar to your PATH. Or use \"composer_install\" to install Composer to current directory."
+  end
+end

plugins/composer/composer.load

+# add Composer's global binaries to PATH
+if test -z "$COMPOSER_BIN_PATH"
+  set -gx COMPOSER_BIN_PATH $HOME/.composer/vendor/bin
+end
+set PATH $COMPOSER_BIN_PATH $PATH
+
+# get composer path
+if test -z "$COMPOSER_BIN"
+  if type "composer.phar" > /dev/null
+    set -gx COMPOSER_BIN (which composer.phar)
+  else if type "composer" > /dev/null
+    set -gx COMPOSER_BIN (which composer)
+  else
+    echo "FAILED to find Composer! Please install composer.phar to your PATH."
+  end
+end

plugins/composer/composer_install.fish

+# install composer in the current directory
+function composer_install
+  curl -s https://getcomposer.org/installer | php
+end