Modify https://secure.phabricator.com/diffusion/PHU/browse/master/src/markup/engine/remarkup/blockrule/PhutilRemarkupListBlockRule.php to support a syntax like
```
#(100) Now the list starts at 100
#(105) this does nothing, because it's not the first node
# a
# b
```
That would render to:
```
100. Now the list starts at 100
101. this does nothing, because it's not the first node
102. a
103. b
```