support for symbols entity

[zboralski]

The symbols entity
An array of financial symbols starting with the dollar sign extracted from the Tweet text.

[nonotest]

Sorry; digging the past.
I recently had to extract this.
Added

Symbols []struct {
        Indices []int
        Text    string
    }

To the twitter_entities Entities type.

if it can help anyone...

[muesli]

@nonotest: Would you mind sending us a pull request?

[nonotest]

Sure thing, will do tomorrow.