As for the first table, you can easily do something like this :
## R table
How do I get this table aligned in the center?
<div align="center">
```{r echo=FALSE, results='asis'}
library(xtable)
tab1<-c(seq(1,10,1))
print(xtable(table(tab1)), type='html')
``` </div>
and the table is rendered in the center.
As for the bullets you can play with html and css to get them to the center :
## Bullet lists
<div align="center">
<div style="width: 60%; margin:0 left;text-align: left;">
<li>And how do I align this in the center</li>
<li>Or just add some spacing on the left?</li>
</div>
</div>
I hope this helps !