feat(context): allow explicit variables to be defined as argument

These changes allows variables to be defined when executing
`kontemplate` via one or more `--variable` arguments.

With this in place one can either define new variables or override
existing variables loaded from a file:

```
$ kontemplate apply --variable version=v1.0 example/fancy-app.yaml
```

This avoids the need to write variables into a temporary file that is
only needed to provide "external variables" into resource sets.

Closes https://github.com/tazjin/kontemplate/issues/122

context/context.go

@@ -12,6 +12,7 @@ package context
 import (
 	"fmt"
 	"path"
+	"strings"
 
 	"github.com/tazjin/kontemplate/util"
 )
@@ -158,3 +159,23 @@ func loadDefaultValues(rs *ResourceSet, c *Context) *map[string]interface{} {
 	// errors here.
 	return &rs.Values
 }
+
+// New variables can be defined or default values overridden with command line arguments when executing kontemplate.
+func (ctx *Context) SetVariablesFromArguments(vars *[]string) error {
+	// Resource set files might not have defined any global variables, if so we have to
+	// create that a map before potentially writing variables into it
+	if ctx.Global == nil {
+		ctx.Global = make(map[string]interface{}, len(*vars))
+	}
+
+	for _, v := range *vars {
+		varParts := strings.Split(v, "=")
+		if len(varParts) != 2 {
+			return fmt.Errorf(`invalid explicit variable provided (%s), name and value should be divided with "="`, v)
+		}
+
+		ctx.Global[varParts[0]] = varParts[1]
+	}
+
+	return nil
+}