bashrc - Add bashrc2 to contain common config

Unfortunately there's going to be a manual installation step to source this
bashrc2 file in the main bashrc. However, there is often a lot of default
stuff in the bashrc that's added automatically that I don't want to either
override or version control. It would be good to add something to the
installation script (in a re-usable manner) to allow automatic appending of
the source command to the existing .bashrc (obviously checking if it exists or
not first, as running ./install multiple times shouldn't append multiple times).
install.sh - Fix bug whereby installing twice will place a symlink of a folder
inside that folder (plus improved logging)

So, for example, installing once would create the symlink ~/.vim -> $DIR/vim,
and installing a second time will create the symlink ~/.vim/vim -> $DIR/vim
(i.e. $DIR/vim/vim -> $DIR/vim). This was because the command
`ln -s $SOURCE $NONEXISTENT_FOLDER` would create a symlink called
$NONEXISTENT_FOLDER, but `ln -s $SOURCE $EXISTING_FOLDER` would create a symlink
>in< $EXISTING_FOLDER rather than failing due to existence as with files. The
fix was to simply check for existence before running the ln command.