findall: support custom function as argument (#488)

* findall: support custom function as arg

```
findall(>(100), cl)
```

docs/src/split.md

@@ -84,6 +84,18 @@ typeof(red)
 ohlc[red]
 ```
 
+The following example won't create a temporary `Bool` vector, and gains better
+performance.
+
+```@setup findall
+using TimeSeries
+using MarketData
+```
+
+```@repl findall
+findall(>(100), cl)
+```
+
 ## Splitting by head and tail
 
 ### `head`

src/split.jl

@@ -22,6 +22,11 @@ to(ta::TimeArray{T, N, D}, d::D) where {T, N, D} =
 ###### findall ##################
 
 Base.findall(ta::TimeArray{Bool,1}) = findall(values(ta))
+Base.findall(f::Function, ta::TimeArray{T,1}) where {T} = findall(f, values(ta))
+function Base.findall(f::Function, ta::TimeArray{T,2}) where {T}
+    A = values(ta)
+    collect(i for i in axes(A, 1) if f(view(A, i, :)))
+end
 
 ###### findwhen #################
 

test/split.jl

@@ -19,6 +19,11 @@ using TimeSeries
        @test findwhen(cl .> op)[2]      == Date(2000, 1, 5)
        @test length(findwhen(cl .> op)) == 244
     end
+
+    @testset "findall(f::Function, ta)" begin
+        @test findall(cl .> 100) == findall(x -> x > 100, cl)
+        @test findall(cl .> 100) == findall(x -> x[4] > 100, ohlc)
+    end
 end