Allow overwriting of default TOC title

[domdoescode]

Currently the Table of Contents title is hard coded. Being able to change this would be advantageous where others may want the project name instead, especially as this is always before the intro contents.

https://github.com/npryce/adr-tools/blob/master/src/_adr_generate_toc#L47

Overwriting with -t or a similar arg could be used to achieve this.

[chchrist]

I think it is better to define a template for the TOCs than just override the title. For example I might not want it as an ordered list. Instead of intro, outro and prefix etc you could just specify a template and pass the variables. If I find some time I will give it a go