bin: rename and move backup excludes file

Use the common dotfile-ignore naming pattern for the backup ignores too.
shell: use `~/.dotfilesrc` for custom `brew`

Rather than messing around with when to load `~/.bash_profile.local` and
always being mindful of whether a custom `brew` PATH has been sourced,
expect the custom path to be set in a new file: `~/.dotfilesrc`. It can
be used for any up-front configuration that is needed, while
`~/.bash_profile.local` continues to be for local Bash overrides and
settings.
shell: explicit export of PhantomJS bin
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