Bash-completion, based on @swansontec implementation.

The `configure.ac` code is generated by `libbitcoin-build` using the following metadata. The implementation is generalized (having no knowledge of bash completion), and required the addition of the `dependency.extract` and `dependency.default` attributes. These provide the additional feature of extracting a package config variable into the option variable with the given (optional) default value.
```xml
<option type="with" name="bash-completion-dir" default="yes" example="[=PATH]" unprefixed="true" substitute="true" conditional="true" description="Path to bash completion directory." />
<dependency name="bash-completion" version="2.0.0" option="bash-completion-dir" unprefixed="true" extract="completionsdir" default="${datadir}/bash-completion/completions" />
```