Add options for customizing the loading bar

[Unixsys]

Steps to reproduce

Consult the man page. As of this writing, there is no option for customizing the loading bar elements (eg. =, >, [, ]) without editing and recompiling the source code.

Expected behaviour

An config that allows people to change the loading bar elements without editing and recompiling the source code.

Actual behaviour

See Steps to Reproduce.

[fanglingsu]

@Unixsys I'm not sure it it's worth to allow to change this via config file or during runtime. So I mark this as request for comment to see what other users think about this.