Add ERB-like trim mode to tag syntax

This is a rebased and updated version of #49. It solves #162.

It performs slightly worse on the benchmark, but not much worse. I think this is important functionality if liquid is going to be useful for email templates, so I'm hopeful that you'll find the slight performance impact to be muted by the value of the functionality for that purpose.

Before:
```
                   user     system      total        real
parse:         4.150000   0.000000   4.150000 (  4.160474)
parse & run:   8.790000   0.020000   8.810000 (  8.830523)
```

After:
```
                   user     system      total        real
parse:         4.570000   0.000000   4.570000 (  4.574668)
parse & run:   9.140000   0.010000   9.150000 (  9.161449)
```