Add operator argument to `where` template function

Add operator argument to `where` template function

It allows to use `where` template function like SQL `where` clause.
For example,

    {{ range where .Data.Pages "Type" "!=" "post" }}
        {{ .Content }}
    {{ end }}

Now these operators are implemented:

    =, ==, eq, !=, <>, ne, >=, ge, >, gt, <=, le, <, lt, in, not in

It also fixes `TestWhere` more readable