bash: Check if in `.git` dir when building prompt

Some of the `git` commands will not work when run inside the `.git` dir.
Therefore, need to test if we're in a `.git` directory and then return
early if we are.

Fix gh-8
osx: Credit @mathiasbynens for OS X settings
bash: Add 'bash_' prefix to custom files

Make the naming consistent. Thanks @alrra
Run `brew cleanup` on update

Uncomment this line of code to clean up old installed formulae. I
haven't had the need to keep old formulae lying around.
bin: Abstract ask-confirm steps

The ask-confirm flow appeared twice (for now) in the `dotfiles` script,
resulting in duplicated code that could be abstracted into dedicated
functions.
Add a new submodule: vim-javascript

Vastly improved syntax highlighting for JavaScript.
Add `vim-javascript` to README's Vim plugin list
Add a note on changes required for forks

Make it clear that if this project is forked, the developer should
modify the variable values in `bin/dotfiles` that contain paths to my
dotfiles. These variables are found at the top of the file and are easy
to change.
Suggest installing iTerm2 and the Solarized theme

The custom prompt mimicks Solarized's colours but for a complete
experience it's best to install iTerm2 and use the Solarized Dark theme
to provide the terminal colour scheme.
bash: Rework configuration and path export

Split `bash_exports` into two files:

* `bash_config` is where custom Bash configuration is set.

* `bash_paths` is where path modificiations are made.

The private/custom settings in `bash_profile.local` are sourced as late
as possible (so that those settings can override any set in the
dotfiles), but not before the `bash_paths` file. This ensures that `brew
--prefix` is known to `bash_paths` if Homebrew was installed in a custom
location.
bash: Minor improvement to comment consistency
bash: Break `bash_functions` into separate files
bash: Make `cleanup` a generic function

Replace the `cleanup` alias with a more flexible function that deletes
all `.DS_Store` files by default or, if a pattern is specified, all
files that match it.
vim: Load `.vimrc.local` last

The settings from `.vimrc.local` should be loaded last so that they can
overwrite all other pre-existing settings in `.vimrc`.
bash: Use consistent function declaration

Make use of the terse, portable function declaration syntax supported by
bash (and other shells) - `func_name() {}`.
bash: Rename `e_ask` function
Update options listed in help text
Various copy improvements
Update submodule 'vim-less' to latest version
Update submodule 'vim-markdown' to latest version