This changes `echoParam` template function behavior to accept not only
an array or a slice and its index pair but also a map and its key pair.
This also changes the function that float and uint values are treated as
a valid result type of it.
Fix#771
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
'where' template function used to accept only each element's struct
field name, method name and map key name as its second argument. This
extends it to accept dot chaining key like 'Params.foo.bar' as the
argument. It evaluates sub elements of each array elements and checks it
matches the third argument value.
Typical use case would be for filtering Pages by user defined front
matter value. For example, to filter pages which have 'Params.foo.bar'
and its value is 'baz', it is used like
{{ range where .Data.Pages "Params.foo.bar" "baz" }}
{{ .Content }}
{{ end }}
It ignores all leading and trailing dots so it can also be used with
".Params.foo.bar"